What Causes Red Swollen Bump On Stretch Marks On Stomach/back?

I have stretch marks on my stomach/back, and I just looked at my back as a joke with my friend, and saw a big red bump in the middle of one of my stretch marks. It's pretty big and red and swollen. What could it be? Ive never noticed it before, but it was a tad bit sore when i poked at it.