They rested for a while after their banging. Brienne was probably sore, seeing that he had just taken her virginity. Jaime didn't really care.
Thoughts of Cersei had made him hard once again and it was dark. In the darkness his mind was playing tricks on him. The longer they lay there, the more his sanity eluded him. To him, she wasn't Brienne anymore. She was Cersei now. He had to have her again.
He was about to have her again, when her voice broke through his insanity and stopped him. She spoke some words, which he didn't pay much mind to. But that voice, that wasn't the sound of Cersei's sexy voice.
That's when he came down from his fantasy and realized. This wasn't his Cersei. This was Brienne. The woman who was like a sister to him. Or rather like a sister was supposed to be to him.