Underwear issues?

I've recently been having problems with wearing my underwear, and I'm not sure if it has anything to do with any sort of weight gain I may have had or not. I went from a size 7 underwear to size 8 last year. They felt fine for a few months, but then they started cutting into me, in my inner thighs like right underneath my buttcheeks, it's hard to explain where it is but its like they cut in and chafe. It gets so raw I can't even walk. I thought maybe they were too small, so I went up to a size 9. I don't have a small waist, butt, or thighs, but I never had. I don't know what changed, but even as I go up in underwear size, no matter how big they are it still happens. It happens mostly while I'm at work, I'm a CNA so I'm walking and moving around literally the whole 8+ hours everyday and the underwear are so unbearable I've even taken them off in the middle of a shift. So I've resorted into wearing my husband's boxer shorts until I figure something out. Any advice???