3 yr old ocassionally vomits mucus?

My son is 3 and has had some small bouts with vomiting mucus. We have an appt with an allergist but want to see if anyone else has experienced this. My wife seems to think it's a food allergy, while I believe it's seasonal allergies (which I have). When he was taking zyrtec daily he wasn't vomiting. We stopped, under dr's suggestion to see if allergies were the cause, in august. He went about 6 weeks without getting sick until last night. He'll throw up food but the end is a lot of mucus. I just can't imagine it being a food allergy because I would think that Zrytec wouldn't help at all with that. Possibly a mild lactose intolerance? I just feel like if it were a food allergery it wouldn't "build up" over time like this seems to. It would happen every time he ate the affected food.