The term pain mountain was first coined in 2008 by a young gentleman in Perth, Australia. It's a saying which suggests that the user is in a momentous amount of pain.
Pain Mountain may have been derived from equally ludicrous sayings such as "I'm on(/in) a mountain of pain right now", although who really knows.
It's a turn of phrase that brings with it a sense of patheticness and the use of this phrase is usually accompanied by sever intoxication and desperation.
*Jesse falls over and sprains his ankle*
Jesse: fuck me im on pain mountain!
Confused bystander: steady on old chap!