Spotting and why ??

So I've been spotting for like two days period isn't suppose to come until next week I had unprotected sex last month only one time, I never had spotting before the only symptom I have is constipation never had that in my life I'm very regular nothing has changed I'm not on birth control or anything. I've been having stomach pains, and nauseous also and that's it and not really eating much could it be pre menstruation or pregnancy?