It's a pretty common. Part of it is just letting up on the stress, which gives you time to finally freak out a little. Part of it is also realizing that you now have no excuse not to go to medical school and work very hard for at least 7 years: the universe had a chance to save you from being an adult for another year or two and declined, and I'm sure some part of you is disappointed by that. Finally there's the reality that you just went from someone who could do anything to someone who is going to be a doctor. The narrowing of possibility, even if the possibility you ended up with was your top choice, is depressing.
The trick now is to do make the time from now to medical school all about you. For this brief period you are free from both responsibilities and guilt: quit your lab, don't study, and if possible do something you've always wanted to do (travel, dive, cimb, whattever). Go celebrate. By the time orientation comes around you'll be back to being stressed and happy again.