The reason described by you could be the skin crease that could have been irritated due to friction.
As you have not noticed or mentioned any other symptoms I do not think this is anything serious. But, if it is indeed friction that has caused the tenderness try using a talcum powder to reduce the friction and watch if the symptoms reduce. But if they do not, then I would advise a visit to the doctor for a physical examination as well as a transvaginal ultrasound
Do not worry ma'am. I am sure you will be fine soon. Be patient and report to your doctor if you notice any worsening of your condition.
Wishing you lots of luck and a long life ahead.