Hello; I am an Audiologist.
Since you don’t mention any significant pain or fever, it may just be negative air pressure behind the eardrum, rather than fluid or infection. You mention a “stomach flu” as opposed to a respiratory flu, so that also suggests to me that this is more allergy related than anything.
Taking antihistamines for your allergies is the best place to start. Allergy medicines with a “D” after them have decongestants in their formula, as well – be careful not to mix medications. If in doubt, ask the pharmacist when selecting over-the-counter medications – especially if you are on any type of prescription medications for any reason. (The same is true for herbal/dietary supplements, as they can also interact with medications).
Drinking hot teas and broths and inhaling steam helps open the upper respiratory pathways, and saline rinses for the sinuses are also beneficial to many with allergies and related sinus/ear issues. Blow your nose gently, as using too much pressure can force air and mucus up into the middle ear space, which can lead to infection, dizziness or even a ruptured eardrum.
As others have noted, if it persists, gets worse – or if your symptoms include pain, fever, dizziness or significant hearing changes – go see a physician.
I hope you feel better soon!