What Could Knots On Labia Majora Suggest?

Hi I am 24 years old and 30 weeks pregnant. I just had an appointment with my doctor and she checked my cervix and everything was normal in my vaginal area. However, last night when i went to bed I noticed a large knot on my labia majora that is painful to touch. It is the size of the grape now and I don't know what it is.