Both my DD and DS have allergies and have been tested and confirmed positive for the following allergies- DD birch, cottonwood, alder, dust, cats. DS-birch, cottonwood, alder, dogs, garlic, and oats. For the environmental allergies DD takes singuliar and flonase and my DS takes zyrtec and nasonex (for the food allergies we have benadryl and an epi pen, which I hope to never use, we just make sure he doesn't injest garlic or oats).
It is very hard to keep up with the upkeep of allergies. Some of my best suggestions that the allergist gave us is 1. cover all mattresses and boxsprings with dust mite allergy covers also cover all pillows. 2. My kids are not allowed to have stuffed animals

3. Wash all bedding in hot water at least once a week. 4) we have 3 air purifiers that run in our home. 5) I vacume everyday with a vacume that has double hepa filter protection 6) we put solid surface floors in their room (allergist stated that the most important room to be allergy free is the bedroom) 6) leather furniture and no throw pillows. 7) wash any curtains monthly (we now only have hard surface blinds that I can wash with soap and water frequently) By doing these things it has improved the quality of life for both my kids. My DD has not had a sinus infection or pneumonia since implementing all of this and my son is sleeping better and his snoring is getting better.
Sorry so long.... I feel for you and hope you can find what is causing the allergy. We did the skin prick test for both kids. It seemed horrible when they told us about it but, it was very simple, not horrible at all and neither kid even cried. DS was on 16months old the 1st time we tested for environmental and 2 when we had him food tested, DD was 3.
I hope you little one starts feeling better soon.