What Causes Red Sore Bumps On Stretch Marks?

Hi, I have really bad stretch marks from my second son. And one stretch mark only it seems, to get a pimple/boil like red sore bump in it. It's happened now about five times. What could it exactly be, and what should I do? It's painful! I've popped them before and it made it worse no mater how clean I kept it.