What Could The Hardened Thick Discharge During Menstruation Be?

Hi I have been having irregular periods y whole life, I have 3 children. My last one was born in June of 2013. I have also been on and off of birth control usually the Depo-Provera shot. Last month, I only spotted, and have had pregnancy tests done through urine and blood that turned out negative. Last night, in the toilet I could see a little bit of pink, but today I had a little bit of regular bleeding, but a lot of blood clots that I would have to say are larger than a dime, but are longer. Just a little while ago, I went to use the bathroom, and I noticed something stringy in my pad, and it was stuck to my pad and it was hardened, after only spending maybe thirty minutes between bathroom stops, and it is about as long as a paper clip that has been straightened, but really thin. Looks like tissue with blood on the outside, but not exactly sure about it. What do you think it could be? What do you think I should do?