4 year old with fever and complaining of aches in legs

[BlueSprite]

We're currently waiting for a call back from out of hours, but in the meantime can anyone tell me how worrying this is? DS has never complained of this before. We're used to high fevers with cold symptoms,but never aches before. He said his back ached yesterday too.
He is eating and drinking, although is subdued (though still conversing with us) and has chills due to his fever.
Feeling anxious :(

Oh, and he had some diarrhea yesterday too. Poor chap.

Diarrhoea, I mean!

I'll bugger off in a minute and wait for replies patiently, but this could be relevant. We were exposed to chicken pox about 2 weeks ago. DS actually had the vaccine (we thought this a good idea due to his eczema), but the child who got chicken pox had also been vaccinated. Hmm.

Maybe he has the flu? Makes you achey? My ds had chicken pox - we spotted the blisters first before he got ill.