It could be an inflamed Bartholin's gland - these kinds of cysts are pretty painful from what I've heard. Any gynecologist should recognize this and be able to lance and drain it. Though, from what I understand these are usually right on the labia near the entrance, which might be different from your specific location.

Here are some links:
http://www.cnn.com/HEALTH/library/DS/00667.html
http://www.mayoclinic.com/health/bartholin-cyst/DS00667

I've never had one of those, but I have had some serious ya-ya acne and ingrown hairs with deep cysts that lasted for months. Maybe you could switch underwear/pant styles for a while to avoid angering it further?