It's not time for my period, so why am I bleeding?

Yesterday, I went to the bathroom and there was a small amount of light red blood on the toilet paper. Twice that happened. So far today I haven’t had that issue, but my nipples are very tender. I ended my last period on September 2, and according to my calculations I shouldn’t be starting again until the 30. Is this just early PMS symptoms or something I should worry about?