I had sex yesterday and my vagina is itching and burning also swollen. What could it be?

Difficult to say. If you had really vigorous sex, you may have caused some irritation to the lining of your vagina. Also some people can have a reaction to the condoms or the spermacide in some condoms. Avoid sex for a few days, soak in plain water baths and keep your genital area clean. Call your doctor if it doesn't get better in a few days.