Underwear issues?

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Recently (about a year maybe a little less), I've noticed that sometimes I find that one side of my underwear(thongs usually) rubs me raw and I have to take a break and wear boyshorts or bikini type underwear until it heals. To clarify, it is on the outside of my labias, like where the sides of your underwear rests.

Is it cause the underwear is too tight? Too lose? Does it happen because of gaining weight?