Somehow she feels betrayed. Her own body, always obeying her every wish and complying with most demanding parts of her job, has gone rogue and turned against Natasha. She knows herself well, most of somatic reactions and actions for her are something she is always aware and able to control, now she looks to herself in mirror and she is in body that Natasha has no idea how to handle, cells, hormones, chemistry working inside her, beyond her control and even understanding.
Because in the end of the day this is impossible. The helth check when she arrived in SHIELD confirmed what she suspected – the Black widows are made to take life, not give life. Some chemical imbalance that none could tell if it was side effect or Red room intention. She believes it is side effect – abortions are easy to get and would allow butchers in labcoats to get hands on some premium genetic material.
When finding out about her infertility, in some twisted way Natasha even grieved. Not that she saw herself as mother and honestly having kids scared her, and not worrying what contraception hormonal injection gonna mess around with was awesome... But still it had hurt a lot to face the reality of even not having a choice. Not to mention the digging through debris of her mind to figure out why it hurt at all and how to seal shut this breach so none can use it against her. Then Natasha picked herself apart and put back together, unnoticed under heavy watch of still weary SHIELD. Just to come undone eight years later when her body unmistakenly signalizes about changes she cannot place at first but then sudden stereotypical carving for pickled cucumbers like ones in Russia makes answer hit her head like full speed train.
Why I feel so happy? Natasha, following old red room protocols, starts self analysis routine with question that stands out the most for her. Once she has replies and understanding, maybe, Natasha cannot help to hope, maybe she will emerge whole again and still wanting to have this child.