Why is my dog dragging her butt and pooping very dry?

i bought her yesterday the guy who was selling her was feeding her left overs with dog food since yesterday she's been dragging her butt and is scratching her stomache alot and pooping very dry small poop she struggles to poop she ate today but didnt finish the bowl i know when a dog drags her butt it means their anal glands need to be expressed but could it also be that the reason why she is doing all of that is because she has worms