Hello and thanks for trusting me to help you and Oma today. I am a veterinarian with over 25 years experience and would be happy work with you.
Temaril-P is often used in dogs who suffer from seasonal allergies and overall is generally well tolerated with little side effects. Since it does contain a corticosteroid however, I would not be able to tell you it is safe to use on Oma without knowing her full medical history and having examined her myself. Only her vet would be able to do that. Steroids are sometimes used to manage IBD as well, but in some cases can be irritating to the GI tract.
I can tell you that there should be no interactions with the medications she is already taking, and I am sure that her vet, who has a better understanding of her conditions, would be able to make that call over the phone if you call them when they re-open. It is always best to check with someone who can review the medical plan.
In the meantime, there would be no harm in giving her a dose of the antihistamine benedryl to see if that will offer any relief until you talk to your vet. The dose is 1-2 mg per pound that your dog weighs so a 20 lb dog would get between 20 to 40 mg. This can be repeated two times per day. The pill that is available over the counter is typically 25 mg. so you may need to go with the children's liquid form which is typically 12.5 mg/teaspoon (or 2.5 mg/cc). Make sure that there are no other "cold-type" medications in it like decongestants
Another option to keep her comfortable until you talk to your vet would be a cool water bath. I am sorry that I don't have anything else to offer, but we want to do what is best for Oma and not take the chance of flaring up her controlled IBD.
I hope this is helpful. My goal is to provide you with excellent service – if you feel you have received anything less, please reply back as I am happy to address follow-up questions. Kindly rate me when you are done. Thank you for allowing me to assist you.