I have been having a problem with some of my running pants creating a lovely "abrasion" on my rump and general area.   The first pair I threw out (okay, goodwill). They were good for 5, 8, 10, mile runs, but when I wore them in a half marathon I got a seam abrasion on my rump (when I inspected them afterwards, realized they weren't princess/flat seams as I had thought).  They were an after thought in the suitcase and I planned to wear shorts, but it was really cold so I wore the knickers instead.

The next happened in my last half in September. These were Nike and have princess seams--the abrasion was at the cotton insert along a princess seam in that location.

The next abrasion happened tonight, in my beloved Nike shorts (the ones with a pocket over the rump). The abrasion is in the same spot as mentioned above.  I'm worried this is going to happen with the knickers that are the same design.

a) does anyone else have this problem and what have you done as a remedy to ensure it doesn't happen again

b) does it help to wear sports undies with the pants?  I figured I'd chafe if I wore those (and had been advised against wearing underpants); however, I just saw UnderArmor at the store and wondered about them, but didn't spend the $$ for them before checking out the question with others.

c) I picked up some bag balm to help with the abrasion :-D