Uncomfortable about fertility

I've been with my SO for over a year now. We use the pullout method and I have never gotten pregnant, well recently my normal 28 day cycle got very messed up and I was 10 days late. When my period did come I was in excruciating pain, like my periods normally make me feel. I have a lot of family members with endometriosis and cysts, some of them won't ever be able to have kids, and I was showing some of the same symptoms as they did. So knowing this I asked to get checked at the doctors. But I'm very upset because I don't want to have anything wrong with me, and I want to be able to have kids later in life. Knowing that I might not be able to have kids makes me cry every time I think about it. When my period was late I told my SO that if I had to chose the reason my period was late I would have rather it be because I was pregnant and not because there was something wrong with me that was detrimental to my fertility. I know that we are young (I'm 19, he is 20), and he has even expressed to me that he isn't sure he really wants kids, and if we ever did get pregnant that he would want it to be after he was done with his service in the marines. Well lately he has been making remarks that really hurt me, like when we were wrestling on the bed one night and I accidently kneed him down there. I was very apologetic and felt horrible for hurting him, his response was "If you keep that up then we really won't be able to have kids" he has made one other remark like this and every time he does it makes me feel really horrible. I don't know if he actually realizes how much I'm going to hurt if I actually can't have kids. He might be trying to make light of the situation but that comment really made me uneasy. Any advice on how to talk to him about this or make him understand that the jokes he makes about it actually just scares me more about my fertility?