I will also add the 'calm down' words going around the board. First, no one single exam will ever make or break you. Even if you do 'fail' (which you won't and I'll come back to that,) this one single exam won't affect your acceptance chances in the slightest. No med school is going to consider a B/B+ in physics 'bad'.
Second, I do this, my wife does this, I think everyone does this. We start to look around at all of the other students in the class ( in my physics classes there were 300+) and we start to think, 'How am I going to do better than all of them?' While forgetting, that 1/2 the class skips class, doesn't bother to do the HW, study, do the lab work, etc... and really has not put in any effort at all. A lot of them are still in HS mode (usually these classes are full of freshmen) that think they had a 4.0 in HS w/o any work, that college can be done the same way. By virtue of simply worrying about it (which I also hope means you are actively studying for the test), you are already putting in more effort than many others.
Third, While I really don't know about your physics class especially, most of these quantitative intensive classes, (i.e, math, physics, physical chem) are set up with just a few (5-6) problems, worth lots of points each. They are not going to do it all or none. As I am sure you are aware, set-up is HUGE! remember that there are 40 or so formulas that you will have to use on the test. Understand them, what they need (the input values) and what comes out (the output) values, show a little math, and you usually achieve greater than 60% of the points on the exam.
Lastly, your not overeacting, your just normal. While I don't ever get as emotional as you said you did about this class, I would be lying if I said I never worried about tests. We all do. You (and me and everyone else) just need to use the stress positively. In the grand scheme of it all, no school will specifically deny you or accept you on the basis of one grade alone. We are all more than just GPA's, or so I like to think.
