My 69yo mother in the uk started getting mentally unbalanced 2 weeks after minor surgery. now she is really bad suddenly. could be brain infection?

Possibly: Take her to a doctor or er. It sounds like she is getting delirious; this may be due to an infection, and not necessarily just in the brain (sometimes, in the elderly, utis can also cause confusion). She needs a proper thorough medical evaluation.

? early dementia: Other new medication may cause confusion if she is still using it. Vs ? Early dementia.