She is 6 years old and in the last few weeks she has lost a lot of weight and no matter what I do she's not putting any on I've wormed her and that didn't help I don't know what else to do

What are you feeding her. My hound cross, (looks like a staffy, vet insists she is part way there :) ) doesn't have the least bit of trouble putting on weight at 7 and she gets IAMS weight control, measured portions.

When you took her to the vet and he suggested worming, what other tests were done? Were worms found in her stool and that's why this treatment was given, or did you do this on your own without consulting a vet?

It could be anything. It could be a blockage, it could be cancer. There's really no way to diagnose online. You'll have to take your dog to the vet, this is obviously not a minor issue.

The thing I think of when a dog or cat loses excessive amounts of weight is the dirty "c" word.. Cancer.

She needs to be seen by a vet. Only your vet will be able to give you the answers you are seeking.