Why are my periods lighter?

Hi doctor,

I am 20 years old. I started to have birth control pill at the age of 15 for heavy periods. I switched to Depo-Provera after about four years on the same pill. I was on Depo-Provera for one year and I stopped it a year before due to irritating light spotting and mood changes. After I began to have BC pills, I get period only once in three months. The spotting now only occurs after sex or self-stimulation. Also, my period was extremely light. I started the next pack of pills, but still experiencing postcoital spotting. I attended a gynecologist for a regular checkup and they called for an ultrasound regarding the light spotting. It was found that my endometrium was at 15 mm. They said it was extremely thickened and required a D and C. As per my research, I found that this is not at all an abnormal thickness for when I am about to start my period, so I disregarded it. During the examination, they found that I had a minor infection or irritation with my cervix and put me on a course of Flagyl. The postcoital spotting stopped completely. A week later the spotting started again. It is now time for my period. I thought that since my endometrium was thickened, I am getting lighter period. What would be the problem?