I don't know if antihistamines raise bg levels, I do think the hayfever would raise bg anyway. I get hayfever from grass pollen from June to August and take loratadine, and use Nasobec nasal spray when grasspollen levels are very high.
I am trying to push back my 12 month HbA1c test so that it doesn't cover the hayfever season. I was diagnosed as T2 after test at end of August last year, at 49. I suspect my reading might have been higher because it covered the hayfever season. When I had my 3 month retest in November it had come down to 44. That might have been due to low carbing and exercise, but it might also have been lower because it covered 12 weeks after the grasspollen season.
I have delayed my 12 month retest for 4 weeks until 1 September this year, and will do that for the next couple of years so that eventually my !2 monthly HbA1c is in December, and unaffected by any possible effect of hayfever (and antihistamines).