Doctor thinks nothing is wrong with me

So since I was sexually active, about a year ago, I've always bled during or after sex. I thought it was normal until one of the last times and the bed looked like a murder scene. (My partner is slightly larger than average) I do realize that bleeding isn't normal, I just stupidly thought it was because I'm barely 5'0 and that I was too small. 
But to go along with abnormal bleeding, I've been having cramps, abdominal pain, and lower back pain. I went in and got tested for infections, which I did test positive for and got treated but this has continued even though I've been treated. I'm still bleeding after sex. I'm still in pain. 
I insisted on an ultrasound because it's happened for a long time. My doctor went along, but kept brushing off my statements because I'm young. So my ultrasound is completely normal, which is great, but I want to know what's wrong with me! 
I got my first period at 10 and I'm almost 20. I had to go on BC when I was 16-17 to have a better quality of life. The doctor suspected I had endometriosis because my periods were so heavy and painful, I was almost passing out. I could barely function. 
So now, the same doctor who thought I had endo (who doesn't even remember me) is completely convinced all of my problems were because of my STI and other infection which were only recently. Even when I insisted this has persisted for much longer. 
I'm frustrated and upset and at a loss at what to do. I know my body and I'm at my wit's end. My stress and anxiety isn't relieved, I'm just even more stressed now with more questions.