You don't mention the vet - have you taken her to be examined yet? If not, it may well be in her interests, especially if this is a constant problem.
Otherwise, the problem could be caused if she's taken, for example, around fields where farmers could have done crop spraying?
Is she usually taken to the same place(s) for walks?
Have you spoken to other people walking doggies in case they've experienced similar problems?
Being a terrier, I'd half expect her to disappear into the undergrowth etc if given the opportunity?
All sorts of things could be causing the problem but you really need to take her to the vet, in case it's being caused by any poisonous substance she could have gotten on her paws, hence the reason she's licking them.