What Causes Pink/brown Vaginal Spotting While HPT Is Negative?

I've been having spotting for the last two weeks. Sometimes it's really light where you can hardly notice it on the toilet paper and sometimes it thicker. It's light pink or brown. A few days ago I passed a blood clot. I also have been getting sharp pains in my abdomen. They generally don't last longer then two mins, and they happen 3-5 times a day. I have a very irregular menstral cycle. My last period was September 28th. Me and my husband have been trying to concieve since the begining of Nov. I took a pregnancy test a week ago and it was negative. Is it possible I'm pregnant? Also, Uterine fibroids run in my family. My mother had to have a partial hysterectomy at age 32 and my aunt at age 30. I'm 21 years old, is this something I should be worried about?