If you're a guy, you've likely experienced it before. And it's really painful. But what's going on? Is something weird happening to you? Should you be worried? Should you be heading to the doctor? Why is it even called blue when there's no blue? And thank goodness nothing changes color down there. Here's what it is.

Blue balls are the pain you feel in your testicles.

It happens after prolonged sexual stimulation without ejaculation. Basically, it's a build-up of blood flow or other fluids.

Testicles not only produce testosterone and sperm, they make other fluid to help propel sperm through various tubes. When there's more stimulation, there's more blood flow to the genital region.

The fluid wants to get somewhere, but the gates aren't opening, mate.

And when that happens, imagine a ton of people pushing at the back, and what you're feeling is the pressure build up.

There's usually no swelling, but the engorgement is happening inside the testicles.

It can be relieved by ejaculating.

Or a lot of rest. You won't need to go to a doctor unless the pain is excruciating.