08 Nissan altima

it ain't the gas cap. you have one or more cylinders misfiring and dumping unburned fuel down the exhaust into the converter(s). damage will occur to converter(s),don't drive it with the CEL FLASHING! TOW to repair shop.

Random thoughts : Did you actually loose the fuel cal or it just turned up missing? ... Would anyone have a reason to contaminate the fuel in your tank? As stated - simply not having a gas cap on the car will not case 'those' symptoms .. *Hands-on* testing by qualified personnel will be required.