I stopped having periods and have gained weight. The pregnancy test is negative. What could be going on? I'm 18.
"Secondary amenorrhea" is the term we use when a woman is having periods and then stops having them. Of course pregnancy is the most common cause of secondary amenorrhea, but all of the other causes are not normal.
A person can have a "post-pill" amenorrhea when discontinuing birth control pills until the pituitary gland resumes normal function after being blunted during the time on the pill, and this is fairly harmless. The same applies after Depo-provera shots and other hormonal manipulations.
But there are other more suspicious reasons to experience secondary amenorrhea. An ovarian cyst (benign, pre-malignant, or malignant) can interrupt the cycle. A benign condition called Polycystic Ovarian Syndrome (PCOS) puts your cycle on hold because you can't ovulate. This interruption causes a build up in testosterone and weight gain.
Of course I'm not your doctor, but if your symptoms were a test question on my finals, I would recommend a serum testosterone level to see if it's elevated, an ultrasound to check for cysts of the ovaries, and of course a thorough physical exam.
If all of this turns up nothing, then it's possible that being overweight to begin with will cause production of extra estrogen from your fat cells, which will muck up the works. And then there is the tragic consideration of premature menopause, which at your age (18) is extremely unlikely. A simple blood test called an FSH could be done at the same time as the serum testosterone.
Because of your young age, I have to figure nothing bad could be going on, but I can't guarantee that. You need to be evaluated by an OB-GYN. Hope this helps.