I use in ear monitors almost exclusively now, which are supposed to save your hearing in theory, but on occasion I've found that certain frequencies will mess with my perception of pitch. I used to encounter this back in the day when stage volumes were too high and sub harmonics would trick me into hearing things in a different pitch a well, but it can happen with IEMs too. I pride myself in having good pitch too, as I'm usually the lead singer too, but I was horrified when I recorded a gig last year on my GoPros and found I was really flat the entire night.
I can tell when I'm flat, so the fact that I had no idea scared me. It meant I wasn't hearing properly, and I've ince played with my IEM mix to give me the clarity and key reference I need.
Are you on IEMs? If not, I assume you're using a floor wedge then? Whatever the case, give yourself a monitoring mix that cuts out all those sub harmonic frequencies that muddy up a mix. Bump up the low mids/high mids and cut back bass. Less is more. It might be ugly or thin to listen to, but if you're amp's going DI (or pre/EQ post/EQ) to the mains and a Soundman, it's his job on the outs to give you all the bass you need. There's really no reason in this day and age to be cranking stage volumes up passed 11.