What Causes Stoppage In Menstruation?

Hi i was just wanting to know something. I havent had a period since July started July 9 ended the 15th, then not another until today.Well im not peeing blood i only get blood when i wipe and its not heavy mostly VERY light i wear pad and nothing is ever on the pad.But when i woke today there was a size make half dollar in my shorts but not every dark. I dont have money or ins. to go to doctor so i was wondering if you could tell me why?