painful nipples

I have been on the pill for years had regular periods and in February I tried the depo shot it caused my period to become irregular I was supposed to have my second depo shor may 13 but decided not because my husband and I decided to start try for our first. anyway about a week ago my nipples started hurting extremely bad and have been hard constantly. I thought it was pms but is the past my entire boobs would hurt. what could be causing this? pregnancy? pms?