Can an ear infection be caused by using a neti pot?

If you see all the sinus cavities from the ear, nose and throat are all connected.

It is highly unlikely (but not impossible) that water from nostrils will enter you ear canal. But yes anatomically since they are connected water can enter your ear from nostrils.

So yes use only absolutely pure and filtered water for neti pot.

Make sure even neti pot is clean and i mean absolutely clean.

Maintain the cleanliness of your neti pot regularly without fail.

Since water is entering your nostrils it has to be cent percent clean.

Even the salt you use should be clean without dust or impurities. Normal cooking salt available is good but ensure its clean.