Could have been a few things, you could have gotten a bit too deep or rough and hit your cervix. Usually that hurts right away in the moment so you'll usually know when you've done it. Or you could have a type of muscle Fibroid in your uterus and it's been swollen so that having sex and hitting that area can cause discomfort, sometimes for a few days. Or you could have over exerted yourself and stretched something too much when doing your BDing, and it's a bit tender and sore. Either way the first and last aren't that dangerous and will heal or disappear with time. The second is something you will need to have your doctor or gyno scan for and isn't dangerous but can be discomforting and possibly a risk in not getting your egg to stick to the lining, but that is rare as well. Also, with the second it would happen multiple times not just once. Good luck hun, and I hope you get the answers you seek.