Many species of monkeys are known for their shit flinging capabilities. Bitches can be anyone of offensive nature. Therefore the bitchmonkey is someone who throws their shit at you just to make you miserable. In the instance of the bitchmonkey the shit is metaphorical for problems, psychological issues, or in rare occasions, actual shit.
"That bitchmonkey ruined my birthday party by having a meltdown at the Chuck-E-Cheese in front of everyone."