You should go to a clinic and get tested. It might be a latent STD flaring up, or just a skin rash of some sort in an unfortunate area. Any break in the skin will burn when it comes in contact with urine, because urine is highly irritating.
Until you get seen, wear loose cotton panties and loose skirts, or loose breathable pants if you must wear pants. No tight jeans. You can take a cool sitz bath to try to soothe it, and a cold pack wrapped in a towel or several paper towels might help soothe it with cold (don't do longer than 15-20 min at a time).
This is a symptom of STD. Do a test for Pelvic Inflammatory Diseases.