It sounds like you are suffering from allergies. My husband didn't have major problems with allergies until a few years ago. Now he feels the irritation I felt through the 20-some years we have been together. I have terrible allergies.
You didn't mention it so I will ask: Do you smoke? If so, this can cause one eye to be inflamed from allergies and not both. I used to smoke tobacco cigarettes and would have that problem off and on. Now, I am off the tobacco cigarettes completely and only use the electronic cigarettes with e-liquid. I've moved completely away from tobacco flavors too. The only thing you inhale with the e-liquid is a water vapor with nicotine in it, not the tar, tobacco, and dust that comes from tobacco cigarettes. I haven't had a true tobacco cigarette since July 7th, 2009. I'm coming up on my first year without a regular cigarette - I just get the nicotine kick and even that has dropped in milligrams since I began using it.
BTW, if the Benadryl isn't working, you can buy the "real" Sudafed at the pharmacy counter. You have to sign for it but you can get the real-deal antihistamine that works better and it's available in 6-hour or 12-hour sustained release. I use a combination between the Sudafed 12-hour and Flonase spray. My eyes stay dry and irritated much of the time but it's b/c I have Sjogren's disease, which causes the tear ducts not to work right as well as the salivary glands in the mouth. I use Refresh Liqu-Gel liquid eye drops to keep my eyes moist and a pill I take, which is usually prescribed for cancer patients but works for those with Sjogren's disease. I also have lupus but the Sjogren's is what causes the irritation in my eyes on top of the allergies.
Hope this helps. If I were you, I'd pick up a pack of Sudafed and give it a go. It might help with the irritation in your eye at least until you can see your doctor.