What Causes Blood Clots During Menstruation?

was on cambridge diet 2 yrs ago which caused me to have a light but constant period for a few months. this stopped when i came off the diet. i am now on slimming world and have been "on" since jan 11th. last 2 days have been really heavy, bright and full of clots. last tampon flooded through after just 30 mins.

anything i can do to help this?