Very well could be flea dirt - although that looks pretty 'big' for flea dirt, unless there is a lot of flea activity. The only way to know is to flea comb her - get some soapy water and some paper towels. Comb the area and dip the comb in the soapy water, then wipe off the comb with a paper towel. Look for fleas and flea dirt in the water, and also look to see if what you wipe off the comb on the paper towel causes it to turn 'pinkish' in color. If it does, that would be flea dirt - which is nothing more than flea poop - and the pinkish tint is from the blood that the fleas excrete in their poop from biting the cat and ingesting blood.
If not that, take a sample of the 'debris' you are seeing to the vets for an analysis - after, of course, asking if they will take samples for testing. It could be from some other parasite.
Does she go outside? It could be from something she gets on her underbelly while outdoors.