I was sitting at a bar alone just eating food and drinking female bartender rubbed my back?

so...the old cashier lady in my supermarket is romantically interested in me because she touched my back? it doesn't mean anything. people do that all the time.

She sees you a regular and sometimes with regulars, whether you talk or not, you sort of obtain a comfortable familiarity, learn to like them. It's very possible it was a way of saying "hi", letting you know she noticed you.

Women, in my opinion, use flirty eye contact and facial expressions first before any touching to show interest. Women are usually affectionate like that in a platonic sense, and it could be if you are a regular customer she is showing a friendly appreciation or may have thought you looked sad or worried at the time.

I think she must have thought you looked pretty down, she probably just wanted to comfort you and cheer you up.

It's a comforting thing, like hey buddy, hang in there, type type thing. Its a nice gesture not a sexual one.