Why Is There Spotting At The End Of Regular Period?

I always keep track of my period, after the last day of my peroid it usually comes back around 18-22 days later. But last month or two months ago I had my period then about 5 days later I was spotting blood for like three days then i got my peroid as usually 19 days passed i have it again now but it has been lasting about 7 days now which is unusual because it usually lasts 5 days on the 6th day rarely VERY light spotting. I do have sex with a condom so im worried because this has never happened to me before and no i do not have sex while on my perois and on no birth control. :)