Pink my guinea-pig keeping has told me that they are the most contrary creatures , and if you are worried about them it's far better to have them checked and heave a sigh of relief than look back and regret missing something.
My DD has a nearly 4yo boar and since last year he's lost and gained weight, he was the dominant boar when we had GP2.
Since GP2 died, we got GP3, had a really hot spell last year, they went on holiday (they enjoyed themselves but it's an unheaval for them). He put on a reassuring amount of winter fat, but now, he's going slim again.
I was looking at him last night, he seems 'bony' his little face is poky and rat like , and he has a couple of the fibrous lumps that they get.
He is eating though, he was first to the supper and the breakfast.
I;m taking him in to have his teeth checked, because although he's eating and poohing, if we are going to have a heatwave, I want him in peak condition - well peak for a saggy middle aged boar 
)
Glad she's picked up though, and have lots of lovely grazing time .
(On a slightly different note, our elderly cat stopped eating, she's purr at the food bowl but only licked her food. We took her to the vets, she had a blood test and started eating the next day .
I did say to the vet "Cats don't starve themselves do they? They are too selfish -in the nicest way "
"Yes they sometime do" he said 
)