I've got a female who gets the exact same marks on her chin. Vet had no clue what the real cause was. I later put 2 and 2 together and figured out, she is doing it to herself. They way she uses her head to try to pry out from the bin, she uses her head like a prying wedge, gets it into a little groove and tries to twist her head. She goes through stages in the year where she is more motivated to try to escape and relentlessly tries.
She has been doing it recently again and they have swollen up. Shes currently about to lay mid june so I understand the angst, she probably wants to find a spot more to her choosing then mine.
She always has marks now under her chin, sometimes they are black(mid process of healing) or if they've healed and had a few sheds it goes almost back to normal but with slight scar/discoloration.Even since changing tubs to something with less of a lip to wedge, she still manages to get them to flare up now and again.
Shes done this for several years now, with no ill effects.
Just my .02. Will post pictures for reference if you'd like
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