From what you are describing I would think it may be a Bartholin's cyst or abscess. The Bartholin glands are in the lower portion of the vagina and while the cyst isn't supposed to be painful, the abscess is and I would imagine if the cyst were big enough it would hurt. You may want to see your gyn about this. Also you can google Bartholin's cyst and you will get a lot of information.
It sounds like what I have. It's a swelling on the right side of vaginal area which hurts when I sit, try to get up, or even walk. The gynecologist examined it and stated that what she thought was a yeast infection was actually a swollen gland possibly infected. Like stated above it could be an abscess or cyst. Other symptoms I have are those you would get with a yeast infection. I was prescribed antibiotics which I need to stay on for 10 days. Also, taking a sitz bath for 10-15 min. can provide some comfort to area. My Doctor stated that if it doesn't cure on it's own.. the area would have to be cut open to drain out abscess. I just pray that antibiotics cures it (I'm on day one) or that It drains on it's own without going to the hospital to get that procedure. OUCH. In any case, don't prolong getting it checked out..I hope you went already. It needs to be treated asap. Oh, and It could have developed from some type of bacteria.