Cat has hayfever

For the past week or so my oldest cat (10 years old), has been sneezing and did have a runny nose. I called the vet and he advised to give her 1/2 of a benedryl tablet (25mg), well, there was no way she would allow that. I tried pulverizing in food, wouldnt eat it, try putting it down her throat, spit it up. She is an outdoor cat. I called the Vet and took the cat in. He said he has seen lots of cats with the same symptoms..hayfever like. He gave her three shots, alergy and antiobitcs. Told me to keep her inside that the humidity was aggrevating the symptoms. I tried to keep her inside, but like I said she is an outdoor cat. She didnt like it one bit. All she did was walk around my feet and head tothe door. I bot a little pan and litter, set her up some food, water, but she wouldnt drink or use the litter box. We went away for a few hours and I had her set up in a spare room, with the door closed. When we got home, I found an old blanket she was sleeping on in that room was wet and had some red where it was wet. Could this be from the shots, one of the shots was reddish in color??? I finially had to let her out and she went directly to the water dish on the porch. She wouldnt drink inside. She wouldnt use the litter box either.. This am she was on the porch and I let her in and she seemed fine but did not want to stay in long, soon she was at the door wanting out. So now what do I do..today is Sunday and the vet is not in, altho I could call him at home. Can anyone give any advise???
There have been other times when Ive had to keep her up( in the barn in a pen I used for my dogs) and used sand in the litter box, which she did use. This time I bot litter, could that be why she wouldnt use the box? I guess today, I will find another box, fill it with sand and keep her in the dog carrier for the hot of the day...The vet said to keep her inside where the AC is and she should be better by Monday...Someone give me some advise...thanks.

I don't know the answers to all of your questions, but the red liquid on the blanket sounds like urine to me...with blood in it. I would call the vet back. Urinary tract infections are quite common in cats and I suspect she could have that going on in addition to the allergy problem.

Yes, I think she might be more receptive to sand in the litterbox. I've brought a number of adult indoor cats inside, either short term or long term and they've all adapted pretty easily to using a litterbox. She might like some potting soil in the box too.

As far as the pill goes, would it be easier for you to administer a liquid? Personally I have a lot of trouble with liquids but some find it much easier. I believe the benedryl is available in liquid form. Some cats can have an extreme reaction to that (dopey, sleepy, etc) so if it were me, I would start with a quarter tablet or the equivalent amt. of liquid. That's the amount my vet told me to use for my 12lb cat.

I use these a lot for pills. I don't have a cat that won't take these...

www.pillpockets.com

...but benedryl tastes very bitter so I would break the pill up into even smaller pieces and put it in 2-3 pill pockets.