Dolly Parton has canceled her previously postponed Las Vegas residency due to health challenges that leave her feeling “swimmy headed” — but has assured fans her ailments are treatable. “The good news is I'm responding really well to meds and treatments and I'm improving every day,” the 80-year-old country superstar said in a short video posted to her official Instagram account. “Now the bad news is, it's going to take me a little while before I'm up to stage-performance level because some of the meds and treatments make me a lit bit swimmy headed, as my grandma used to say.” “And of course, I can’t be dizzy carrying around banjos, guitars, and such on five-inch heels — and you know that I’m going to be wearing them,” she joked. “Not to mention, all those heavy rhinestone outfits, the big hair, my big … uh, personality.…