It sounds like she had a URI. URI's are definitely contagious, so you should watch your other piggies for any symptoms of illness.
Guinea Lynx :: URI Guinea Lynx :: Common Mistakes Guinea Lynx :: What's Normal?
Another thing you should do, if you haven't already, is get yourself a good digital scale and weigh your piggies at least weekly and keep track of it. Weight loss is usually your first sign that something is wrong and it gives you a chance to get your piggie to a vet before it's too late.
Guinea Lynx :: Weigh Weekly!
If you happened to get this piggie from a petstore, URI's are common problems. If you adopted, way to go! If not, please consider adopting if you do decide to get another piggie. Remember to quarantine just to be sure.
I am sad for your loss. You obviously loved your piggie very much.