Brown discharge

A few weeks ago I had a UTI. I assumed it was just a period because I was expecting to start. I treated the UTI and had my next period, although it was a week late. When I was on my period the bleeding was very irregular. I would bleed and then not at all and it was like this for over a week. Well the bleeding has stopped but the past couple days I've been having Brown discharge on and off. I know it's not implantation bleeding because I took 2 pregnancy tests and they both came out negative. Online it said either pregnancy or premenopause. I'm only 18 so I highly doubt ita menopause. what else could this be? I'm getting really anxious about it all! Please help!