Unbearable pain after sex.

I'm 19 years old and up until today have had very pleasant sex with my boyfriend of 2 years. We live together and sex is frequent and no red flags have come up until today. Around 6 this morning I surprised my boyfriend with a quickie before I had to go to class. The sex itself was literally quick and painless. There was no elaborate positioning and it was actually the most gentle sex we've ever had. It was done in like 2 minutes. I quickly got up so I could go catch the bus but when I stood, I was overwhelmed with literally the worst pain I have ever felt in my entire life. It was a sharp shooting pain in my lower abdomen. Before he and I had ever had sex, we would just fool around and tease each other and I'd find myself with the slightest abdomenal pain. We used to joke around and call it female blue balls. It was easy to blame the quickie for this pain. I thought nothing of it and headed out the door. However, before I even got out of our apartment complex, my knees locked and the pain intensified to an unbearable level. I thought I was going to vomit. I could barely even walk. I limped back to our apartment and ran to the bathroom. I was incredibly nauceous and felt the urge to vomit, but whenever I tried I could only spit. My boyfriend tried to take care of me, but just the sound of his voice (or any noise for that matter) made me feel incredibly ill. To even move was impossible. I had to crawl from our bathroom to our bed because standing up was impossible. I couldn't help but cry. My boyfriend insisted I try to sleep and just wake up in time for my next class. When I finally woke up, it was 8 hours later and I had incidentally missed all 3 of my classes. The pain has lessened but it is still there. My boyfriend is worried that it was something he did wrong, but I'm positive this is something internal for me. I have no history of sexually transmitted disease (nor does he) and I am in good health. What is the issue here?