Guinea pig sick?

Question
I have a 1.5 year old female guinea pig (she's almost 2 rather), and I noticed a bit of a change in her yesterday afternoon. Yesterday morning I feed her as usual before I start my day, I make sure she gets her veggies, small amounts of fruit, and refill her hay and pellets in her cage. When I got home that late afternoon, I noticed she was a little lethargic. She felt and looked a lot thinner than normal (she's usually a chubby girl). And she wasn't really getting excited when I was shaking her hay bag.

However, when I did put some more fresh hay that afternoon, she was eating it slowly, kind as if she was bored of eating it, but still eating.
I also attempted to give her vegetables and treats, and she hesitated to eat them at first, but within a few minutes she began to eat everything (at first it was slow, then back to normal pace).

I keep her cage in my bedroom and every night I usually hear her rustle around in her hay and always chewing/nibbling on something. I did not hear anything last night, as if she was just laying down in her bed area all night.

This morning she was still laying down, but when I put in fresh hay she began to eat it again, and when I brought vegetables for her she ate all of it as well.

I'm not sure why she's acting differently, but it leaves me worried because I used to have another pig along with her, but he passed away due to NOT eating at all (about 5 months ago).

Thanks.

Answer
The most difficult thing to do is to be able to find a reason for a sudden weight loss and/or appetite. What she is exhibiting are signs of illness but unfortunately she can't tell us why. One of the first signs of trouble is the appetite, as you've already experienced with your other pig.

I'm afraid I have no real answer for you but can only advise you that the best thing to do before she gets any worse is to take her to your vet. She's trying to tell you that she doesn't feel well, and guessing at this kind of thing seldom has a good outcome. So please Jessica,  take her to be examined hands on.