Vaginal swelling

I am almost 31 weeks and for the past couple weeks now my vagina has been swollen like really swollen. It feels like there is a huge lump about golf ball size. Tonight me and my husband had sex and after I went to the bathroom and cleaned up and it's now probably the size of a softball. The swelling is just around the opening not the whole thing if that makes sense. Does anyone know why or what is the cause?