A/N: I've had most of this on my computer for ages, so I decided to just finish it up and publish it. Set between seasons 1 and 2, with spoilers from 2.1. I do not own The Flash, or any affiliated characters.
Caitlyn had a secret. It wasn't a bad secret, necessarily – she hadn't killed anyone or broken any laws because of it – but the very thought of sharing it left a pit of ice in her stomach. She knew she should just tell – Cisco was her best friend, Barry would understand, Doctor Wells could surely help her – but something inside her screamed that she had to figure this out alone.
She would have told Ronnie. If he'd been there, she wouldn't have hesitated. She'd wanted to tell him from the moment she realized he was still alive, but she'd never gotten the chance. Barry and Cisco were right there, and she barely had time to say anything before he was flying away, wreathed in flames, and wasn't that little power ironic. When he came back for real, Professor Stein ensured that they were never really alone, and then there was too much chaos, and then he was gone again.
Like everything else in her suddenly fantastical life, it had started with the accelerator. After the explosion, Caitlyn began to feel cold all the time. No matter how many layers she wore or how high she cranked the thermostat, she couldn't get warm. Her temperature was normal when she checked for a fever, so she assumed it was just a manifestation of her grief over Ronnie and ignored it.
The first time she tried to cook for herself after the accelerator and Ronnie she stood in front of an oven set to 375 degrees for almost four hours as the frozen pizza stubbornly refused to thaw. She blamed it on a broken appliance and ordered Chinese.
It wasn't until almost three months had passed, and her job had shifted from "theoretical physicist" to "personal physician to some police officer in a coma" that she pulled out of her grief enough to notice that something was seriously wrong.
She was testing another sample of Barry's blood when Cisco called her over to look at one of his new toys. "It's a heat ray," he explained, a slight edge to his usual enthusiasm that Caitlyn only noticed because of how well she knew him. He was doing his best to act as upbeat as usual, but Caitlyn could tell that he was as broken up about the accident as she was.
Cisco kept talking, but she tuned him out in favor of examining the device.
"Careful!" he cried, as her finger slipped. She dropped the device in shock and felt something slam into her gut. "Are you okay?"
She looked down; there wasn't even a mark on her sweater. She was distracted from wondering about it by a much more wonderful realization: she was finally warm. Caitlyn hadn't realized just how cold she had been until she wasn't any more. It felt amazing.
"Caitlyn? Caitlyn!"
"Yeah, I'm fine." He studied her critically, but decided not to press. He picked up the heat ray from where she had dropped it.
"It must be broken," he decided. "If it had gone off it would have burned a hole right through your chest."
"It must be," Caitlyn agreed. She knew that it wasn't. Something was very wrong with her.
Now that she was paying attention, she noticed other things. Although her internal temperature was normal, nothing she touched ever warmed up. It was like she wasn't giving off any heat at all; in fact, she was actively absorbing it. She bought an electric blanket, realized that it didn't warm her enough, and bought three more. Her heating bills increased dramatically. She thought of telling the others, but Wells was clearly preoccupied, and she didn't want to worry Cisco.
By the time Barry woke up, she'd resolved to just ignore it. She was relieved when she realized that she wasn't alone, but the fates of Clive Mardon, Danton Black and Bette San Succi convinced her to keep quiet. When Snart and Rory kidnapped her, she almost laughed. She was scared, of course, but not of Heatwave's weapon. She was afraid of what they would do when they realized she was immune, of how Barry and Cisco would react to realizing that she had hidden something so important for so long. In the end, it didn't matter; Snart called him off, and she returned to Star Labs without exposing herself.
She wasn't sure how long she could keep her secret, but there was always so much happening that it was easy to hide, and no one paid enough attention to ask. She wondered if she should tell after they found out about Wells – cold could stop speedsters, right, and wasn't that what she was? – but there was never an opportunity, and it wasn't like she had enough control to weaponize it or really help without just getting in the way. She knew she was just making excuses, and as everything kept falling apart she wished she had just told them sooner. Maybe if she had, she would have been able to keep everything from going so badly. Barry wouldn't have had to go to Snart for help transporting the metahumans, they could have stopped Wells earlier, without Eddie having to—
Maybe, if she hadn't been so afraid, if she hadn't hidden it from everyone for so long, Ronnie would still be—
She knew that she couldn't change what had happened, and even if she could it wouldn't fix anything (Barry and Eobard had certainly proved that), but when she looked around STAR Labs she felt sick, and empty, and far too cold, and she couldn't bear to stay.
No one at Mercury Labs asked her why she kept the thermostat so high.