What’s going on

I’m 3 months post partum and I got a IUD in November on the 23rd and we started having sex on the 18th of December and recently I had cramping and bloating and around when my period supposed to come (January 15th) I was cramping and spotted for 3 days and then today I had more. Cramping and I wiped and saw more spotting today .. I’m nauseous and boobs hurt and so tired . I took a test 3 days ago and it was negative but I looked at the test again and it was so light I think evaporation line.