Sexual problems that have been going on for four months?! I'm 16, please help me!! :(?

Hi everyone. I'm so frustrated about what's going on with my stupid body! :( I'm 16, I've been having sex with my boyfriend for 10 months and we have been together for almost a year and a half. My problem is, quite frankly, it hurts!! :( ...And it didn't use to. When I lost my virginity I went through a little pain, but I even had an orgasm that very same night. For a month or two afterwards I had no penetration problems whatsoever. Sex felt amazing and I couldn't wait until we had it again. My boyfriend and I used to be long distance up until 5 months ago when he came to live with me and my family. So, there was a 4 month lapse in our sex life. When we reunited, we had amazing sex, once again with no penetration problems. Then... Ever since February, it hurts SO BAD for him to enter me. :( It's ONLY when he enters me, too. I go through a sharp, piercing, ripping, burning pain, and then it stops. It lasts for about a minute, then my boyfriend continues and it feels normal again. EVERYTIME we have sex this happens though! It's so frustrating! :( I've been to the doctor and the doctor said he didn't see any vaginal cuts that were visible, I'm not on birth control (we use condoms), and we both do not have STDs. This is ruining our sex life!!! I'm crying right now. We also use a lot of KY lube too, so lubrication isn't the problem. My boyfriend also can not go deep either anymore because he bumps my cervix. I didn't use to have that problem either! Ugh, what is wrong?! :(