In most cases, the acute onset of a tender mass adjacent to the anus is the result of benign inflammatory conditions such as thrombosed (clotted) hermorrhoids, perianal abscesses, anal fissures, and anorectal fistulas. However, it is, of course, not possible to offer a specific diagnosis to patients over the internet. Therefore, it is best that you see your physician for a proper evaluation (there is really no cause for avoiding a necessary visit with your doctor due to embarrassment issues).
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com