"Dude my piss stream just froze completely solid mid piss! It's colder than a bitch in heat!" and your friend says, "No shit it's colder than a bitch in heat This shit is hella Montanan yo!"
A bitch in heat literally means a female dog who is in heat. (Heat meaning she is 'in season' to mate and have babies.)
A bitch in heat is sometimes used when reffering to humans, generally females. A female ththat goes after every male she can find, and is being extremely lustful. A blunt way to put this would be a horny woman.