Welcome to HCM.
There might be some cut,abrasions or infective inflammation in the vagina after sex or due to improper hygiene.During passing of urine ,it may touch this and causing pain and burning.
Go for investigation like cbc,urine,pap smear
,usg abdomen to rule out the diagnosis.Take proper local care.keep this part clean with medicated soap.
Get examined by gynecologist and start treatment accordingly.