This is common query so it must be a frequent occurrence for contact lens users!
Red and irritated eyes are a common but non-specific symptoms that especially if they occur in contact lens users may be a cause for concern. The causes are manifold from benign causes like dry eyes, a minor scratch of the cornea, to contact lens overwear or even early signs of a microbial infection.
If there are obvious red-flags like decreased vision, significant pain out of expectation, or if there is a visible spot or lesion on the cornea, then you should seek immediate medical attention from your Ophthalmologist.
If the symptoms are limited to that of mild red eyes that do not cause significant pain, and vision is not affected with no visible lesion on the cornea, then a prudent approach would be the use of your own preservative-free lubricant eye drops, stopping use of your contact lenses and observing for deterioration of your symptoms. If the symptoms fail to resolve within 24 hours, or even worsen after usage of your lubricant eye-drops, you are advised to seek immediate medical attention from your Ophthalmologist.
If in doubt, even a short-consult with your Ophthalmologist can help to guide your prudent use of contact lenses in order to minimise the incidence of complications. You can even ask whether refractive laser surgery would be suitable for you if you are increasingly contact lens intolerant.
