Katy Perry Roars Like Crazed Fan ... at Crazed Fan

Katy Perry rarely gets like this ... going ABSOLUTELY BERSERK meeting a fan.

Katy -- who looked ridiculous with post-breakup hotness -- freaked out Sunday, moments before heading inside the iHeartRadio Music Awards. Why? She met the super fan who runs the IG account, Katy Perry Rares ... which has more than 70k followers.

She had to admit ... "you know my life better than I do."