I would take a test to be sure. Getting bigger in the stomach is not likely do to being pregnant at this stage. One month is not really enough time to have the weight gain, so it is most likely a food issue. Sounds like your body is just going through changes, but it never hurts to check.
I'm going to a doctor Monday....and I've been feeling like this since my period got done and me and my BF had unprotected sex...i can't fit in my clothes anymore either...my back hurts and my stomach growls alot alot....i never felt like this before....
best advice is to take a test..then get help whether you are or aren't. you could just have a bug but only a doctor could determine what it truly is. since you are a teen it is possible your body is just changing.
good luck