I agree with BG that it's extremely important you see a doctor. You can have a really yucky kidney infection that will NOT get better on it's own and could land you a hospital stay if it's not taken care of. Tubal pregnancy, chlamydia, there are a number of things both minor and major this could be. But there is no way to know without seeing a doctor. I know that's not the suggestion you wanted, but really, what other suggestions are there? We are not medical doctors here and would be doing wrong to "diagnose" you over the internet without the credentials to do so.
Go to your mother...and say something like the following: "Mom, I'm feeling really bad, it's progressively getting worse, its influencing my school, and I'm getting really really worried. I need to see a doctor as soon as possible." if she says "Oh you're fine", respond "No, I'm not. Something is wrong and I can either go and get it taken care of or end up in the hopsital with thousands in medical bills. If you're not going to take me, I'm going to talk to my school nurse and she will.".
Your parents cannot deprive you medical care. And medical care is definitely what you need.