I have not had this occur to me. But I can say I know it is not unusual to have discomfort from sitting because hips can get sore, my talebone gets sore cause I broke it a few years back. I would say if it is hurting consider calling your Dr. and looking into it, in case it is something that has an easy fix. Good luck.
I would definitely say call your doctor as well. I know that when I get really bad yeast infections my vagina gets very swollen and sore.
If it feels like a lump on the side of your vagina it could be a lymph node, which could indicate an internal infection.
Either way, you will need to speak with your doctor about it. If you are not comfortable with showing your male doctor, you IMMEDIATELY need to find a female midwife or OB; your doctor is going to get a first-row view of your lady bits when you're in labor, so you want someone you are completely comfortable with.
But whatever you choose about your care provider, you need to get this swelling seen to.
I have been to see a female doctor she said I have a abcess and thrush, she says its common I'm not sure if it is or not but she has give me cream for my thrush and some antibiotics for the abcess she says it should go down in a week and if it does not I have to have it drained, but it has gone down loads now and I can sit down without it hurting, thank you for the advice I'm glad I can talk to some people who have been through this themselfs as most of the people I talk to about it have not so thanks for the advise x