What Causes Nausea And Stomach Pain During A Period?

I came off the pill over a month ago and my body has always been like clockwork even off the pill. I was a day late for my period so I did two pregnancy test and both were negative. My period came a day later but I am in so much pain, I'm tired all the time and experiencing nausea. Also instead of being slightly brown in colour from the lining it is just red which is very unusual. Could I be pregnant?