There can be several causes. For myself:
That's what my hands, feet, ribcage, (or vajayjay! sniff!) does after several hours of contact with latex over several hours to several days. I have a very SLOW but severe latex allergy. Underwear, lingerie, socks... anything with elastic in it the effect is much slower when mixed with fabric, but my skin breaks down, and then picks up various infections (yeast or other fungal infections, most commonly), and then the "trenches" form. It takes several days to weeks to get bad ((first starts off as just redness/ "hot" feeling (my mum said I had "hot feet" my whole life... well, not once I moved out!), then itching, then burning, then the skin starts blistering/ sloughing/ callousing/ cracking and getting infected)).
So I avoid latex like the plague (CONDOMS are latex!!! Oy!), most SOCKS and underwear and lingerie contain latex elastic.
Sports bras I just have to suffer the fluorescent red ITCHY BURNING line around my ribcage, but I shell out for silk and lace from france for everyday bras, and skip underwear (or wear spandex or more silk and lace), and wear 100% wool socks or nylons whenever possible, nitrile gloves only, etc.
((HINT: if you're allergic to your socks, wool socks will feel MUCH cooler than most cotton socks over the course of a few hours... although if it's a latex allergy THIN cotton socks won't affect you as much as sport socks or fashion socks... because super thin /sheer socks use nylon instead of latex elastic. ALSO buying all new cotton socks every 3 months -and tossing the old ones- helps a great deal, because the cotton hasn't worn away as much from the elastic, so your feet are in less contact with them.))
I would SUSPECT a slow acting allergy like mine. The cause could be latex, elastic/rubber, cotton, or leather. Since you're feet are either wrapped in them or pressed on them for hours at a time, with heat and water to speed up the reaction you'd see the reaction there before anywhere else.