pregnancy with IUC???

I have had the mirena IUC for two years now, but shortly after getting it put in i lost my health insurance so i have not been able to go back to my doctor since getting it put in for a check up or a papsmear. I have in the last 20 days gained 15 lbs, i have always weighed 100 lbs and my boobs have also grown two cup sizes in the last month. I am worried that i some how got pregnant...as i havent been back to the doctor in the last two years i guess its possible that my IUC could have fallen out and i didnt know, but i also have NOT had a period since getting my IUC and still do not have periods, so i cant go by the old "i missed my period" trick and i also dont know how its possible to get pregnant if im not having periods because i thought that meant that i wasnt dropping eggs. I dont have insurance still, but have applied for state assistance (plan first) so that i can go to the clinic and get tested for pregnancy. I took two at home pregnancy tests and both were negative, but i thought that maybe the IUC could be throwing off the test and giving me a false-negative result due to the hormones it releases or something like that?? I dont know if thats possible but it is the only thing i can think of. I also realize that it is dangerous to become pregnant while having an IUC implanted as it can cause birth defects and or serious complications with a pregnancy. If you have any information regarding this or just tell me what you think or if youve heard of this before please let me knw. thanks!!