Hello everyone

Wanted to see if you can help me with something on dec 26 i got my period i was suppose to get it last month on the 23rd I'm 10 days late & i still haven't gotten my period. When I was 6 to 7 days late i decided to take a pregnancy test but it came back negative i haven't taken another one since. Today morning I started feeling really neasous didn't really eat anything I have lower back pain and abdominal pain as well and I been having discharge. What can it be?