help please!

My period isn't due until tomorrow but starting yesterday, I started spotting. There was bright red blood and then some brown discharge. There's almost no chance I'm pregnant because my boyfriend and I always use protection (condoms). We had sex the night before I started spotting and I've also been stressed lately (college student, midterms). Is that what could've caused me to spot ? I usually never spot and I'm getting nervous. Help please!