Being a g.pig...that is always our first ruleout. Many of the problems that develop with these guys are nutritional or environmental.
The one thing I can think of is called "Guinea Pig Paralysis Syndrome" ...starts with mild fever and sporadic urination, followed by weight loss and posterior paresis (hindend weakness)...then progresses to paralysis. the cause is unknown, but the condition appears nontransmissible. Long term survival is poor.
Let me know how the vet visit goes.