[Toeee]

I am 18 and I have been experiencing what seems like an abnormal peroid. I started what I thought was my period 4 days early but it was very light,pink or bright red, and seemed more like spotting then after about a week I found, half way through the day that it had stopped. The nextmorning I woke up and I found that I was bleeding again this time a lot more heavily and a darker colour, with small black lumps and was rather stringy. It was heavy for about 3 or four days then became a little lighter but still heavier then the first week. Now not much bleeding shows up on a pad however some does and when using the toilet is is very apparent that I am still bleeding. This is my 18th day of bleeding and it shows no sign of stopping. My periods are usually very regular and lasting 6- 7 days. I had unprotected sex with my partner, who has no sexually transmitted diseases, about three weeks ago but once I began spotting I took 2 home pregnancy test and were both negative. I sometimes feel tired and sometimes light-headed and am sometimes getting slight random pains in my left rib/chest area and sides and feel nauseous a lot of the time. Why is this?