Treatment For Irregular Smelly Period

I have been having irregular periods for over 2 years. I have a period 1 month then not for 2 months and so on. The periods I do have only last 3/4 days and are the usual heavy day 1 then light. I havnt had a period for a couple of months then I had sex with someone and 4 days later I started bleeding. I thought it was my period but it hasn t stopped for 21 days now. It s not heavy and most of the time I can only see blood when I wipe myself but it smells really odd and very unpleasant. I have bleed after sex in the past but only for a few days. Also I have never had smelly periods or any problems with odour