Guinea piggies hide feeling ill or hurt, sadly. But you know what... 6 years is a good age for a guinea piggy :) I know it sucks, but it happens. The best thing for you to do, is give her what you can!
My rant: Why is it I am seeing more and more people medicating their fish for silly things? I don't even like medicating my fish if they have a wound (unless the wound is uber deep) and I have never had a problem with clean warm water being my medication for them. :/ It's like people getting 100 flu shots a year whereas I haven't. If absolutely positively needed I will go to a doctor. But otherwise, au naturale for me! (and my fish)
Breed for the breed, not for the money; the words any REAL breeder would understand.
You must know nothing in order to know all, for all is nothing and nothing is for all.