What Causes Menstrual Odor?

At the end of my Feb period noticed a strange smell, then again towards the end of my March period the blood was grey in colour and the stench was awful. Today I have started my period for this month and although the blood is nice and red the stench is foul and i'm a little alarmed. I thought I had a urine infection 3 weeks ago due to the smell from it somy doctor gave me antibiotics however I think it prob wasnt a urine infection now! Laura