First time I rode with a helmet light, the extra weight of the light meant the helmet was slipping down a bit. So I tightened it up, but being an idiot, tightened the chin strap instead of the adjustment mechanism.
Next time we stopped, I took the helmet off and then almost fainted as the restricted blood flow through my neck suddenly returned to normal.
So if you’re as dumb as me, maybe you did the same thing? Otherwise, as others have said it’s probably just getting used to it.