Hi and welcome!
If you are worried, please see a vet and do not home treat on spec.
Whenever you are worried about the food intake, please switch from weighing weekly to weighing daily at the same time. If you notice a loss of weight of over 50g/2 oz in a short time, please see a vet; on the same day if the weight loss is 100g or more. If a guinea pig is listless, it should see a vet. Lethargy and full refusal to eat count as an emergency. Any stumbling piggy sould also be seen by a vet.
The crusts on your eyes look like they have been caused by watery fluid, not by the thick mucus that is caused by a respiratory infection (and I am sorry to say that a bit of vitamin C is not healing a bacterial respiratory infection, which can kill or cause long term damage if left untreated,
@BlueVictorian
).
Watery fluid is the sign of an irritation or damage to the eye, often from something stuck under a lid or a draft (if you have A/C in the room). If you see it for quite some time and the eye looks smaller (drawn in) compared to the other, then it is painful and it should be checked. It can just be a little hay stuck under a lid or a non-ulcerating hay poke.
We can help you better if you please added your country, state/province or UK county to your details since we have got members and enquiries from all over the world. Please click on your username on the top bar, then go to personal details and scroll down to location. That will make it appear with every post you make and allow us to tailor any advice to what is available, doable and relevant where you are. thank you!