Should I be worried?

Okay so I went grocery shopping for my mother since she didn’t feel like going, I had to get gas anyway so she asked me to grab what was on her list.

I grabbed everything checked out and it came up to 280 dollars and now that I’ve dropped off the groceries and asked for my money back she said “ I’ll do it later, you don’t need it right now anyway.” And am I wrong to be mad at her for it?

I don’t think I’m getting my money back at all, there have been many occasions where it was “ added to my tab.” And she still owes me that as well, I’m willing to just get the 280 and be done with it. But I don’t know how to go about asking for it without burning a bridge. I told her before I went I would need to be paid back and she said okay, but now she’s changed her mind it seems.