Originally British English, it's used in various English-speaking countries, understanding may vary. She's English, so saying something cheeky might put her back up.
Pro Tip 2/3
Emotional Response
The phrase often describes a defensive or indignant reaction, rather than just annoyance. When accused unjustly, that's likely to get his back up.
Pro Tip 3/3
Social Context
It's used in informal conversation and may be inappropriate in formal or professional settings. At family dinners, avoid topics that could get people's backs up.