re: Just met the GF's mom- should I be concerned?

That's a tough one. You never know what is going on with her mom. She could be facing some serious challenges in her life that you know nothing about, which could be causing her to be not like herself. A death of a loved one, menopause, loss of a job, infidelity. You just never know. I'd keep your feet and mind firmly planted in the relationship between you and your girlfriend without letting outside things (like her mom's mental state) interfere with that.