What Causes Heavy Cramps With Blood And Brown Colored Discharge?

i am 18 years old 5ft 170 pounds. i had my period a week. it was normal. now every 3 hours i have heavy cramps along with blood and brown colored discharge. it only lasts for around an hour. i have been taking birth control pills for 6 months and this has never happened and have not misses any pills. i have had a miscarrage over a year ago.