Pregnancy while on birth control

Ok idk what's going on wit me right now. I have the iud for birth control, I have used this for like 2 years now. I'm like clock work when it comes to my periods, but they r very light and for about 4 days. We'll this month I'm about 5 days late from starting but I did spot, very very light, three different times in the past 8 days but not 3 consecutive days..... I did take a pregnancy test 4 days ago and it was negative. Idk what's goin on. Has this happen to anyone before?