Vasocongestion.

You may not notice it but the testes swell to nearly twice their size just before orgasm. That's blood rushing into them to get them working overtime. Usually that blood drains easily but if you've been turned-on for a while it may drain more slowly leading to blue balls. If you're really horny though, the prolactin you secrete to cause your genitals to deflate may not be enough to do the job and keep your genitals at least partially congested.