Do you think some could just be anxiety? It's pretty normal to feel different somehow after having sex for the first time, and anxiety can cause weird stomach feelings. I definitely felt weird for a couple of days after.
I'm very happy to hear that you used protection. :) Very smart. Even if you hadn't, and you got pregnant, you wouldn't feel anything yet. It would be far too soon to have any symptoms of pregnancy.
Are you using anything for birth control besides condoms? Maybe that would help you relax about it. You don't mention your age - which is fine and we don't need to know - but in many states, you can get birth control without your parents permission. You can go to Planned Parenthood and get them, or there are online sites now, like Wisp and Nurx. You have to have a credit card for the online sites, so if you don't, try Planned Parenthood.
Birth control doesn't protect against STIs, so continue to use condoms, too.
Hang in there, okay? I'm sure it's going to be fine. Just be gentle with yourself and know what you are feeling is normal. :)
And you wouldn't feel pregnancy this early. It might be lingering excitement or arousal or just feelings for the person you had sex with. Sex, especially when it's new, can be a pretty intense experience both physically and emotionally depending on the person, especially if it was something you were wanting to do for a long time and finally did.