Is it possible to arouse a fixed female beagle just by petting her stomach?

My sisters beagle is 13 months old and she's fixed. We correct her when she misbehaves. My sister also spoils her rotten. When I do punish her, she is submissive and for the most part relatively obedient. Last night she sat by me on the couch and I started scratching her stomach. Within moments her breathing became labored and she tried to hump me. I freaked out and everyone in the living room started laughing, me included. Why would she try to establish dominance when she knows I'm the boss? Was she just stressed and trying to communicate something with me or was she just what appeared to be, ?