Could I be Infertile?!

So my boyfriend and I have been together for a little over three years now. Although we aren’t trying for kids yet ,because I have yet to finish school, we aren’t too careful when having sex. I’m not on birth control and we hardly ever use protection. I would have thought that in these three years we would have had a pregnancy scare by now. Not that I’m in need of one but after so long I just think it would’ve happened. And I know it’s not him since he already has children. So could it be that I’m infertile or is it just our luck?