She probably thought either 1) you were a needy nuisance - after all, you took up extra time, with your request, or demand, that someone fetch your baked beans - you took up more time and attention and resources than other customers - and required special service - or,
2) that you were a condescending, entitled prat. There are many ways of saying thank you, some of which come across as genuinely grateful and respectful, and there are others which come across as entitled, patronising and condescending.
Either way, she probably wished you on your way and away from her till as fast as possible.