Poor wee Rosie!!
She is giving you very little to go on as to the underlying cause that has triggered what are advanced stage emergency signs of distress in the guinea pig. Therefore, if her breathing is laboured then this needs to be addressed promptly.
Now we can see breathing become laboured as a side effect of (1) GI pain (or pain in general) or with (2) respiratory (lung based disease or pneumonia -- ie bacteria, viruses or mycoplasmas) disease.
To complicate the picture, you have noted this anorexia. Even on its own anorexia is very serious in this species (it is one of the few emergencies for this species). Guinea pigs are designed to be eating all the time, as they have a gastrointestinal tract that is like conveyor belt and needs to be moving/digesting all the time. And if he isn't and his gut has ground to a halt, this can lead to serious consequences. Namely, the bacteria naturally in the system overgrow and release gas (painful!!) and toxins, which cause a pig to get even sicker and less keen to eat or drink.
In this situation, you really want to have a vet see her today (I wouldn't wait). The vet will be able to examine your lass, listen to her lungs (and determine if there is a pneumonia present) and provide oxygen if she is really struggling. If they localize the problem to the abdomen rather the respiratory, then pain relief can be administered to help him while they determine (ie foreign material, obstruction, infection/inflammation, etc) and address the cause. Depending on the findings, they will be able to help you approach this and give Rosie the best chance of recovery.
While you are sorting out getting her seen, do consider keeping her warm (so she doesn't have to expend further energy to do so). A good way to do this is to use a clean sock, and fill it 2/3 with uncooked white rice. Tie it closed and microwave (approx 1-1.5 min). Make sure to shake it before adding it to the cage, to allow the heat to distribute. Make sure its not too hot (as we don't want to burn the pig. If it cools, you can re-warm as required). This is quite a good way to keep them warm in their carriers en route to the vets.
Overall, laboured breathing and anorexia are two serious red flags in the guinea pig that we must take seriously and address as soon as possible. As I am sure you are aware, this is a species that will hide illness as long as they can, so this abrupt deterioration is quite worrying that we are looking at either a late stage respiratory based infection or that she is suffering from an acute painful GI based issue. Therefore, in this situation, I wouldn't risk spending time on home care and monitoring, and strongly advise having a vet listen to his lungs and check her belly. The sooner you pin point the exact cause for this startling decline, the sooner you will be able to address it appropriately and get her feeling better.
If you don't already have an exotics vet, you can find one near you at http://www.aemv.org/vetlist.cfm or http://www.guinealynx.info/vetlist.html.. If you are struggling also check here (http://www.rabbit.org/vets/vets.html), as rabbit vets often see our wee pocket pets as well.
I hope this information is helpful.
Please do let me know if you have any further questions.
If you have no further questions, feedback is always appreciated.
All the best,
Please remember to rate my answer, so that I may receive credit for my assistance today. IF you have any lingering questions or concerns, please reply to me via the REPLY or CONTINUE CONVERSATION button with the questions or issues you might have. I will be happy to continue further and do everything I can to provide you with the service you seek. If you are satisfied, please click the 4-5 stars or associated happy face so that I may receive credit for my assistance.