Lost My Virginity and the Condom Fell Off!

So yesterday I lost my virginity to my boyfriend after dating for a year and a half, it was really romantic and sweet and I'm happy he was patient and understanding to why I waited that long. We used a condom, but once he came into it he didn't hold onto it as he pulled out, so they condom slipped off when he got soft and it stayed in my vagina. I'm really nervous that some of the cum could have gotten in me, but he assured me that everything stayed in the condom and that it just slipped off as he pulled out.

I'm just wondering if this has ever happened to anyone before? Did he grab the wrong size condom or should he have just held onto the condom when he pulled out? Also, before anyone says anything about birth control, my body reacts negatively to added hormones and I've gotten life threatening blood clots before from birth control (I used to use it when my periods were irregular). So my birth control options are pretty limited, and my mom is very against pre-marital sex. I'm thinking of getting a copper IUD, but right now my main concern is making sure I'm not going to get pregnant from this accident.

Any help is appreciated, thank you!!