CatSitting...cat acting strange. HELP?

So me and my roommates are watching a friend's cat and she's acting strange. The first day she seemed fine, she was eating, exploring, purring etc. She followed us around rubbing our legs and everything. That all changed this morning. She had jumped up on the stove and pooped (which its owner said she wouldn't do) and I sat down and was scratching her head behind her ears. She was stretching and purring. All of a sudden she dug her claws into my hand and bit me, growled an ran under the love seat. Any time we try to approach her-or even LOOK at her-she hisses and growls and takes clawed swipes at us. What's going on here? And how do we calm her down? I don't want her owner thinking someone hurt her cat because we've been nothing but caring and affectionate. And I was only petting/scratching her head in the spot she likes so it doesn't seem like she got pissed because I touched a tender spot on her. I'm really freaked out. What do we do?