Look at the meds you are taking. Some meds can cause you to gain weight or not lose weight. You might be able to change to a different med (talk to your dr). Another problem might be about sleep. I know that when my son's allergies are bad he doesn't sleep well. Not sleeping well can cause you to have trouble with your weight. Figuring out how to manage your allergies so you can get good sleep might help. And I guess a final problem might be more psychological, but still allergy based. If food doesn't taste good (because you are congested) or if you are feeling terrible (because of allergy symptoms) you might not be eating properly. I know I rarely eat well when I have a cold and I figure it could be the same with allergies.
