Ask an Eye Doctor and Get an Answer ASAP
It sounds like an allergic reaction. It may take longer for the antihistamine to work. I usually do recommend cool compresses. You can take another antihistamine in 6 hours (unless you have to work). Otherwise, if it does not resolve in 12 hours, you may need steroids.
The other thing it could be would be an infection (from a stye or insect bite). These are usually red and hot.
Benadryl seems to work best for me; although it can make some people drowsy. Claritin works better on chronic sinus problems than on acute allergic reaction like what you have.