I've been called selfish because I chose drinking over helping out a friend and I don't know if I'm truly in the wrong here?

My friend has been asking me to go down and help her out by bringing her grocery shopping. She doesn't have a license (she is 31) because she is terrified of driving so she is very much reliant on literally everyone. Usually I don't mind helping her out but I'm done this week. It's only Wednesday and I've had a fuck of a week. She lives an hour from me.

So basically I got home from work a half hour ago and I decided that I'm going to have a few drinks. Picked up a 12 pack. I've had one. She calls me like ten minutes in to me being home and asked if I could bring her today versus Friday to grocery shop because apparently she has nothing in the house to feed her kids (she has food but her kids won't eat it because they are picky as fuck). I told her no, I was drinking tonight. She originally just laughed, assumably because she thought I was joking and when I said I wasn't kidding, she started begging me. I told her no again and stood firm on the fact that I am drinking tonight. I'm not doing anything else.

She says she isn't mad at me for saying no but is mad at the fact that I chose drinking over helping out my best friend and says that it makes her feel less important to me. I don't know how to go about this because I'm a fucking adult and if I want to drink, I'm gonna drink. The plan was for Friday. Like yeah, I get that she has kids who don't want to eat what's in the house but I also don't see that as my problem either. How do I go about this without showing her how pissed I am (because I'm mostly pissed about work, not her)?

I've been called selfish because I chose drinking over helping out a friend and I don't know if I'm truly in the wrong here?
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