Old blood after period!?

My period is usually super regular and nothing ever seems to seriously effect it, not even stress. However, the last two periods I have had have been five days later than normal, (my cycle is never more than 31 days and its been like 36). I just had my period last Friday to Friday and everything was normal other than it being late. Now all of the sudden I started bleeding just barely again this past Wednesday night and yesterday (Thursday) I felt a huge gush of blood, there was a lot and it looked slightly old. I continued to bleed off and on yesterday and now am bleeding again today (Friday). The last time I had sex was the last week of May (with a condom, I am not on birth control.) I have no idea what is happening as this has never happened to me before, (bleeding old brownish blood.) I have bled mid cycle twice before though just a tiny bit for a day when I first started my period years ago! Any ideas!??? Thanks.