only if you wacked your head... poly styrene on the inside of most helmets, once compressed does not rebound... this was standard practice with motorcycle helmets... even if you dropped it from a distance of about 2ft of the ground...
now, that being said, its your call, statistically speaking, it'll probably serve you well for a long time to come... myself, I'd probably keep using it for a while but would be mindful that a new one was in order during the off-season and plan accordingly...