I really shouldn't give you this secret...after yesterday's game
rnntn
My nephew had the same problem in our 5K today...but that was because he wears basketball shorts that ride up on him.
My son ran cross country and track in high school and college...my daughter played tennis in high school and college...a lot of chafing between legs and under arms...I carried a tin of Bag Balm in a back-pack to every event...I was quite popular with the kids, especially in hot weather! They were always coming for a "scoop"! Not sure of other brands but the kids....especially college tennis girls...swore by it!
Do you wear the "wick it" moisture away brands of clothing? That seems to help also.
Happy un-rashing! ~ Gary