16 daya till next period, extremly sore boobs and cramps. Is this normal or am I pregnant?

Im 22 years old. Ive had the mirena IUD since I was 18. My periods for the last 2 months have been just brown and starting to get lighter and lighter each time like my IUD is finally kicking in and stopping my periods. Well last period I had the brown for 4 days a little red but not much at all I normally go anywhere between 9 to 12 days so it was short but now that its almost 2 weeks after my period my boobs are extremly sore, ive been having frequent heartburn, and I just started cramping today. I took an at home pregnancy test and it was negative but according to glow I still have 16 days till my period. Is this normal? Or is there a chance Im pregnant?