Could this be a miscarriage?

Hi I am looking for some information on what you may think is happening with me. I had my last period starting the 27th of October. I did not have one in November or December and have yet to have one in January. Tonight I have been having pain in my lower back and abdomin and when I went to the washroom I had some mild bleeding (what looks like red tinted musuc) and a passed a peice of something (i think it was some type of tissue) that would be about the size of a toonie (yep, I am canadian!) and in the shape of a kidney bean. I am still experiencing the pain and bleeding and have a low grade fever of about 38 celcius. A little more info, just to be sure you are filled in. I am 23 and the mother of 2 boys. (3&1)I have had a NOVA-T IUD in place as my form of birth control for 8 months now. I took a pregnancy test two weeks ago and it was negative, this being said, I did not test positive with my second son until I was 6 months pregnant (hormones don't rise like most...even with blood work)
I wondering if this is a miscarriage and whether I should see my doctor? Any advice is appreciated!
Thanks!