Cramping and spotting mid cycle. What could It be?

Today is day 18 of my cycle(which is usually 31 ish days). I started getting cramps this morning which have continued throughout the day. I have had a little bit of spotting and have felt nauseous. My period has NEVER come earlier than 26 days and I've never had spotting in the middle of my cycle. My BF and I have had unprotected sex. So my question is could this be pregnancy related like implantation bleeding? Or could it early  period? Really not sure right now thanks!