Hello I Am 16 Years Old I Got A UTI

Hello I am 16 years old I got a UTI in the month of February .. From there I went to the doctor to get some medications .. I still took the pregnancy test the week after I had sex and three weeks after I still took the test because I was very very worried couldn't even sleep .. Today when I went to the washroom I noticed a brown discharge on my panty I said it couldn't be that I am pregnant because my results said I'm not .. I also felt severe cramps in my stomach .. I just need some advice and know what is the cause of the is please :(