What Causes Spotting Despite A Negative UPT?

I don t know what to do this is going to be alittle TMI so basically my boyfriend and I stopped using condoms about 3 months ago and did the dirty ALOT we are young in love etc. & I actually have a disease called IC bladder disease that is normally found in 80 year old women I m 19... and I have 2 doctors who are very aware of this I have already had surgery I m sure I will need the surgery again but I thought I was pregnant in the middle of February I had a normal period then towards the end I had a 2 day period then in march my was suppose to come the 16th my period started on the 13th at first it was spotting which I swore I ve never had before then it turned heavy with cramping for two days then on the third day lightness and now barely no blood I did research on the internet some say infection some say implantation? I m confused and stressed I was on birth control but I haven t taken it in the last 2 months my birth control did make my periods go weird before but I ve been off of it for awhile i honestly don t not know what to do and I know it has nothing to do with my IC bladder I know stressing never helps but that is what is happening I also took a test two days before I started spotting it came out negative I m lost please contact me as soon as you can thank you doctor!!!!