Skinny German Shepherd?

Roxy turned nine back in September, but she has been awfully skinny for the past few years. Lately it seems worse than usually. She has long fur, so it's hard to tell unless you pet her. She's probably around 45 pounds. She hardly eats her food without us putting something in it to make her want to. And even since she was probably around three or four, she goes out to poop a lot.

Is it actually normal for a dog of her weight to have to go poop four or five times a day, if not more? Yesterday she had diarrhea. She's going to the vet today at 11, so we're hoping for at least some result (although the there hasn't been in years). Do you guys have some weight gain tips for her or have anything I should have the vet check for?