the heart guard doesnt cover a couple of worms that she could possibly pick up!!
I use interceptor. It prevents additional worms that heartgaurd doesn't, such as whip worms. I had Penny on heartgaurd. When i got Daisy she came with whip worms (which are very contagious-through saliva, i.e. sharing a water bowl). i switched them both to interceptor. When i asked my vet why this isn't reccomended over heartgaurd she said that heartgaurd is more popular because it is a chewy (interceptor is a tablet). this is not an issue for me because mine will eat anything! but, if it was, you could just hide in a piece of cheese or something.

Oh, whip worms can also be hard to detect. the vet didn't catch daisy's on her first deworming but they were most likely there at the time.

I'm keeping my fingers crossed that you can find out what the problem is and begin treatment to get her feeling better.