Is OTHER wrong in the post ? POST : My partner took my son over to his grandparents over the weekend to give me some space as I work very hard both at my job and in the family home and never get any time to myself. My partner is often very ill and I'm often looking after them both when she's feeling worse.

My son was not brilliantly behaved over the weekend at his grandparents, but I got him a small gift (£8) as I missed him so much over the weekend. My partner was extremely, immediately angry when she saw it and began telling me off for undermining her authority as a parent and spending money we don't have. She was really nasty and it turned out her parents had spoken to her about her finances over the weekend (which are not great). My finances are pretty secure and my job is safe. 

I'm genuinely confused as to how in the wrong I am here. I kinda get the undermining thing, but then that wasn't the intention and neither of us often get him treats so it felt like a one off. AITA? 