Discomfort after Sex?

So I had a hookup the other night, to be honest with you it wants not great or even good! I just did it to make the person feel good about himself! Well I made sure that we used protection even though it kept slipping off. But I made are that everything was good and no accidents that would get me pregnant. But I'm just wondering if it is normal to have mild discomfort after having sex. I was thinking it was because I had really engaged my diaphragm. Because that's where the pain seems to be coming from , but it really doesn't hurt is just something like I over worked that muscle! So should I be really worried or just think of it like I have been working out really hard ? Please let me know what you guys think ! Thank you!