when you say "I'll let you know", do you actually mean you'll let me know?

I've been hit with the dreaded "I'll let you know" text, the one that feels like just a nice way of saying no. A bit of context, I asked the girl I'm talking to whether she'd like to go for a drive or have a movie night this Friday, after college ends, to which she asked if she could let me know due to the fact she's ill and she may not be feeling up to even go to college yet (she is genuinely ill mind), but the phrase "I'll let you know" seems like a flake, or a nice way of saying no. So, when you say "I'll let you know", do you actually mean it?