What Causes Brown Greyish Discharge For Person On Pill?

my girlfriend (16 turning 17) height around about 165cm weight in her 40's, found some thick brown greyish, white stuff on her undies, before intercorse, shes on the pill and her period isnt suppost to come. she had a miscarriage recently, and we're affraid and scared that she might be having another one? what could it be?