The change of seasons could be causing allergic symptoms. Benadryl is fine to give, but you need to give it on a regular basis to determine if it will help. Probably you won't notice any improvement for 24-48 hours. You can give one 25 mg capsule every 4-6 hours for antihistamine relief. If after you have done this over the weekend, and you see no difference, then it probably isn't going to help. I would consider seeing my vet on Monday if the gasping continues, as there are things other than allergies which can cause the symptoms you're describing. Some may require prescription medications.
I hope this is helpful. Please push ACCEPT and thanks!