What Causes Delay In Periods And Stomach Cramps?

I got the copper iud in 15 months ago. everything was fine at first but now my period is late. i had a little bit of blood the other day but nothing more since then. i also keep getting stomach cramps, vaginal pain and lower back pain. took a pregnancy test last week and it was negative. so is this normal or should i see someone about it ? also im 20 and i have a two year old daughr.