Other causes are diabetes and hypothyroidism. Electrolyte imbalance too can cause nausea. IBS, gluten or lactose
intolerance, parasites in gut too can be the cause. A stool test should be done for worms, giardiasis etc. Also if nothing is found then food challenge test to detect food intolerance should be done.
Please consult your PCP for primary examination followed by proper referral.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!