What could this be?

Today I went to the bathroom and when I wiped there was a bit of red blood. My period is not due for another week and I am usually very regular. I put a tampon in just to be sure. About an hour later it only contained a bit of pinkish/brown blood and then that was it. Nothing else. During that time I also had an ache in my lower back . I am just wanting some opinions on of this it prementral spotting or any other ideas. Please and thanks.