In women, it feels a lot like the mildest menstrual cramp, or rather, the warm, heavy, vaguely achy feeling that often presages the onset of a period. It's like being made aware of your internal organs, which are sending out "Over here, pay attention to me!" signals. It is not remotely like a kick in the balls, or ovaries, or whatever. Discomfort and frustration are not pain, and even pain does not necessarily signal damage. Blue balls may be safely ignored. Most often, they are ministered to by their owner, in private. Go to it.
Of course, there is such a thing as dangerous emergency-level dangerous vasocongestion. This one is pretty much a boys-only affair. It's priapism, when an erection refuses to abate after a reasonable amount of time and a good-faith effort. This can blow out your blood vessels and leave you limp for life, but since it never ever happens just from some girl sitting in your lap, we can forget I ever even mentioned it.
Don't forget to read Andrea at Carnal Nation.com.