Could my dog be allergic to grass or have hay fever? His eyes keep swelling up!?

Ok, so this morning I let my great dane outside to go to the bathroom when we got out of bed. He was outside for approximately 3-4 minutes before he came back inside. I noticed that he was kind of acting goofy and rubbing his head into his dog bed and pawing at his face. Shortly there after I realized that his eyes were swollen up and had bumps or hives on his head. There was no swelling or hives anywhere else on his body. This was around 6:30-7:00 am this morning and most of the swelling had gone down by the time I got him to the vet at 9 am. The doctor assumed that it was a bug bite or a sting, as I am sure any vet would have due to his symptoms, and just gave him a benadryl shot and no steroids. We came home and he slept for several hours. I let him outside to go to the bathroom after he woke up. He was outside for only a couple of minutes before he came in. About 5 minutes later I noticed his eyes were swelling up again. I gave him 100mg of benadryl as the vet had instructed me to do. This was around 4pm and the swelling went down. I went outside with my dog tonight, (7:45ish), to play with him and keep an eye on him to see if his eyes would swell up again, and they did. I was watching him pretty close and I didn't see anything sting or bite him. I am so confused to what could be doing this to him because all of the "seasonal" allergy related/ hay fever stuff I have looked up seems to go along with more of itchy skin and feet - Not swelling of the eyes and hives ONLY on the face. I will keep an eye on him for the next day or so, and if it continues to happen I will take him back to the vet. I am trying to get advice from other dog owners who may have had similar issues with their dog before I take him back to the vet. I spent $100 for today's visit and I'm collecting unemployment at the moment, so money is tighter than I would like it to be. If anyone has advice or might know what this is, PLEASE help!! And please don't just tell me to take him to the vet, I have common sense.....THANK YOU!!