What Causes Intermenstrual Spotting?

I am a 27 soon to be 28 year old woman and for the past to months I have been spotting. In febuary I had my last semi regular period it lasted eight days instead of it's usual four to five. In the first month that I missed it spotted lightly for two to three weeks. Now this month I have been bleeding very heavy off and on for the past two weeks. Also after six I am also bleeding for a short time but it is a very bright red.