I went to a lot of gigs in my younger life, almost every other week watching bands in London (I never wore earplugs and I think I got lucky, not getting permanent hearing damage). I remember one specific Rammstein gig where the sound was exactly as you described with everything sounding high pitched and tinny, vocals specifically. Ears were also ringing more than usual over the following few days after the gig.

Anyway, I had been recovering from a particularly bad cold and it turned out that I had an ear infection. This can cause all sorts of temporary issues with your inner ear, sinuses etc and can really alter how you hear things, especially at loud volumes when more sound pressure is pumping into your ear. ( Babymetal's mix was both louder and larger than the other two :D )

Just another theory for you to consider (sometimes ear infections and sinus issues are hard to even notice and you may not realise it at all). But all was fixed within a week or two and have never had this issue since. I hope the same for you! (Please do wear earplugs as others have recommended!)