Here's the contact details for Royal Canin

Royal Canin
United Kingdom and Ireland
Crown Pet Foods (Ltd)
Oak Tree Meadow
Blackworthy Road
Castle Cary
Somerset, BA7 7PH
0845 300 5011

I normally like to use email when getting in touch with places, but I found that with most products the number they have listed provides the most help. If you have any concern about the food at all, I'd definitely call and let them know. They'd be able to send you out some postage paid envelopes to send the food to them in so they can run a lab test. As it's your pet's health in question it's probably wiser to phone them so the ball gets rolling faster.

Definitely let your vet know what's going on as well, it could be any number of things. I hope they feel better soon!