This doesn't sound like something caused by giving Ivermectin.
Have you ridden her in the last couple of days? Put on fly sprays or any other topical product? Have you checked her temperature?
It wouldn't be a bad idea to call your vet after checking her temp and seeing how she looks in the morning. You can discuss exactly what you are seeing with him and decide if you should have her seen.
Having been dewormed 6 months ago with an adult horse it's unlikely that you are looking at a heavy parasite burden. Contrary to the old way of thinking and doing things, about 1/2 of the adult horses will control intestinal parasite infections on their own with minimal deworming (twice a year). You can check out this link: www.thehorse.com/Video.aspx?vID=183
It takes about an hour to watch and is really in-depth and detailed on the best way to plan a deworming program for adult horses. Deworming 6 times a year is more than necessary even if you have a horse that doesn't have a good resistance to parasites.