Could be caused by things as diverse as a reaction to changes in detergent, sanitary pads or even things like diet or a move into a different area.
Perhaps a reaction to a change in temperature or clothing
Perhaps a reaction to condoms or sexual lube.
Perhaps a urinary or another infection. Having only one partner is not a guarantee against an STI (or STD) or the host of things that can affect even a nun...
It is very early to give an opinion - just follow the rules and concentrate on cleanliness and perhaps some nappy rash type cream for a week and if it hasn't improved or has got worse then seek medical advice. Meanwhile don't have sex or if you must use a condom...