I don't know what this feeling is. I've never experienced it, but that's because I'm dead on the inside - The feeling of wanting to have children. Some people have ascribed it as a hardwired desire, that seeps into your thoughts and makes you genuinely believe, even if you rationally convince yourself otherwise, that you want to have kids. This is all anecdotal, of course, in regards to women I believe this really kicks off somewhere around their mid 20s.
It's a very powerful thing, strong enough to override past convictions, even. I've seen it happen to women, who were seemingly dedicated to their careers or education, I've seen it happen to men, who had passions and things they would dedicate all of their time towards. Gone, all of it just met with 'haha, yeah, I guess I was like that once, huh?'.
Stop and think about it. Really contemplate it. Where'd this desire come from? What does it stem from? Why was there a change that made you suddenly want to have children? That thought, was it natural? Or is it a result of something innate, primal? You don't want to have kids because you want to have them with somebody you love, you just WANT to have them. Doesn't that sound fucking weird?
Let me put it another way. There are children, right now, who don't have parental figures. Adoption. You can go out and apply to adopt a child. Wherever you're from, this process is probably thoroughly documented online because they WANT people to adopt them. Very likely you probably don't view that option nearly as enticing as having your OWN children, do you? If this is what you mean - then sure, why not, go for it. Not everyone's cut out for it, make a new thread with that premise in mind or do your own research to figure out if you can do it.
If it's not the latter, then it's all in your head. Ignore it. Every single day, hour, minute and second that it occurs to you. Ignore it. Any pain or negative emotion that it causes you, ignore it. Your boyfriend's gone.