What Causes Light And Dark Vaginal Bleeding While UPT Is Negative?

Hello, I have been sexually active for about 2 years. We do use condoms and even the pull out method to minimize the likelihood of pregnancy. I m 18 years old, and since i started my period i have had fairly irregular cycles. I tried tracking but its never predictable, though i havent skipped a period until now since i was in my early teens. In the month of April i had very light and dark bleeding for a few days. (Not even enough to soak completely through a pantieliner)however i did not get a period or any symptoms of a period in May. Worried the bleeding could have been implantation bleeding, ive taken 4 urine pregnancy tests since a week after the slight bleeding in April, all about 2 weeks apart. All have been negative. Now, June 22nd, I m experiencing very bad cramps, exactly what period cramps feel like, and have noticed similar light but dark spotting. (More brown than red in color). I have an appointment next week to see a doctor, but my most recent symptoms alarm me. For a few days i experienced constipation and now i have minimal diarrhea, not crazy enough to send me running to the toilet more than once a day, but it is present. I honestly don t believe this is pregnancy, but im not sure what else it could be. I am not any more or less stressed or active than normal. I haven t gained or lost weight, etc. Any ideas?