If the sex is "rough" this can cause irritation or even swelling. STDs that can also cause swelling areGonorrhea & Chlamydia.
It may not mean that any disease is present. Even when I have slow steady sex the constant rubbing sometimes causes my vaginal area to swell. Also remember that humans are wired for sexuality and when one gets aroused blood flows to the vaginal area making it swell a little. Of course this is quite obvious in men for without the blood flowing he wouldn't be hard. If this is a constant thing it wouldn't hurt to get checked out by your doc or by a free clinic.