Why Am I Not Having My Menses From Past 6 Months?

I haven't had my period for about 6 months and i'm only 21. i've also been having a lot of chest pains that are extreme sometimes. My left eyes twitches sometimes and my back pains are more often then before. I want to a hospital in september and was told I wasn't pregnant and that it was nothing. I have a hard time believing that because I have a hard time breathing sometimes, I'm gaining alot of weight, and I can barely sleep well. Please help!