Got my period 3 times this month!!

The 4 on January got the peorid lasted 4 days days normal heavy bleed for me. Then on the 17 I started bleeding agin and it lasted 3 days medium flow light cramps I thought it was weird then to day the 31 I started bleeding and have mild cramping and back pain. I'm not pregnant haven't had sex in December or January. Had a pregnancy test done at the doctor for a check up earlier in the month. What could be wrong with me. I did stop take depo in August. I'm allitle scared.