2005 Nissan Xterra FUEL SYSTEM ('CHECK ENGINE SOON')

I filled up with gas one day, & after traveling approximately ten miles, my 'check in soon' light popped on. Later on that afternoon, while sitting with the engine idling, suddenly my 'low fuel' light popped on too (the tank was still full.I hadn't traveled much after filling it up earlier that day). So now, I have the two lights going. All the while, the engine continued to run fine. No changes at all in the performance. At first, I thought for sure it was bad gas (rumor has it that water has been known to be in the fuel at this particular station), but I've been filling up there for YEARS without a prob.
My husband disconnected the battery terminals for a while (in hopes of re-setting the system), lights went off, but temporarily. Cranked up the next morning- 'check engine' light was back on. After a couple of days, it went off by itself and stayed. UNTIL I filled up again- at which time the engine light came back on as soon as I cranked back up at the pump. This is now the 'routine': 1. Put gas in the tank (little or a lot- doesn't matter) 2. Crank up, and 'check engine' light lights up. 3. After a day or two. It goes back off until I fill up again. In the mean time, my 'low fuel' light will randomly come on too (AND, sometimes, when the 'low fuel' light comes on, the fuel gauge NEEDLE will bottom out on 'E'- but not always.)
Its CRAZY! Got any ideas? Thanks for your time and assistance!