My seasonal allergies cause nausea and fatigue. are there any ways to treat this?

Unlikely: Aside from food allergy or protein enteropathy (when the food protein is toxic to your system), people don't get nausea from allergic diseases. One exception may be nausea from a profuse amount of post-nasal drip although I have yet seen one. I would look for another cause for nausea. Fatigue however may be linked to allergic rhinitis.