Muskrat up your butt

In this novel I'm reading I found the following question by one of the characters:

"Are you a local peacenik with a muskrat up your butt about the military?"

Can anyone tell me its meaning? Also, is it a spiced-up version of a more common expression, like 'to have a rat up one's... ' or something similar?

Thanks a lot!

About the same as "chip on your shoulder."