Here is a photo of the red bump on scrotum
I'm not a doctor, but that looks like a blood blister.
It's hard to tell because you a sort of pinching it, but your doctor will be able to tell. It does not look anything like an STD.
Oral sex is pretty low risk for STDs, and the STDs you'd be at risk for from that don't produce symptoms that look like that. You had protected sex, so I wouldn't worry about STDs at all.