Apologies if I'm missing the point but despite your drip feed of
- Not having the money
- Bursting for a wee
- 3 heavy bags
- In pain from all the above
You were fine with being in the shop looking.
I sympathise that to go back and just miss it to someone who (as far as you've decided) doesn't have as an emotive and deserving back story as you is annoying. But it's just life.
Not a dick move, perhaps if she could have mind read the very detailed level of deserving of the bag you have she might have given it to you.
But she didn't and of course couldn't so just bought something she wanted. Hell maybe she liked it because her dog had just died/it hadn't died it had just eaten her changing bag/her toe hurt and it made her day.
She had no idea what was going on in your head and didn't give you a second thought. Hate to break it to you but this is called the human condition. Just minding her own business and buying what she wanted with no thought at all as to how it might impact some stranger behind her.
Don't get upset with posters telling you the truth - that she was a total stranger in a charity shop buying something she wanted in a charity shop.
For all you know when you were shopping the day before you inadvertently bought the last loaf of bread/apple someone had their eye on/whatever. Did you spend your time wondering at the possible impact this may have had on random strangers after you?
Were you being a dick then? No. You were just going about your day utterly oblivious all the back stories/feelings/emotions of all the people around you. Like everyone else in the world does when they are out shopping.
It's fine to be annoyed. It's even fine to blame random oblivious woman and seek validation on here that she was a dick for not being able to sense for your very being that you deserved the bag.
But stop bristling at being told it was just the way the cookie crumbles sometimes and it let it go. Can't be good for you.
I predict there will be other changing bags. Maybe even a nicer/cheaper one.