Confused, could I be pregnant...

My period isn't due for another two weeks. Almost right after my last period ended I continued to have sharp cramping in my uterus on the right side, this has continued through now (I actually woke up in the middle of the night because it got so bad). I've had off and on nausea and headaches. Today I've had horrible upset stomach, off and on nausea and heartburn. I did take a pregnancy test, even though I haven't missed a period yet, it came back negative. My boyfriend and I of 5 years have always used the pull out method and have never gotten pregnant, he hasn't ejaculated in me but there's still that feeling of possibility that I could be pregnant. Im not worried of being pregnant, just concerned and curious if these weird symptoms sound like pregnancy or something else' I'd appreciate if anyone has any input or opinions.