I am 19 and got my first period at 14. Since that time I have never been what you call regular(28 days). I average normally around 5 week cycles. This past year since I have been in college I have become very irreguar sometimes as short as 3and a half week cycles to as long as 7 week cycle(one time recently). I had my last period the night of 8/13 which was only 18 days from the one before on 7/26. This is the first time that it was that short in between. It was not heavy but I am on day 8 now and am still spotting this morning. I am not sexually active. Is this normal or should I be concerned?

Actually, that sounds normal to me. As long as they still last usually at least around 3-4 days and they aren't excessively bad, it should be fine.