1998 Hyundai Elantra check engine light on

Last week my boyfriend fulled my car with gas, about half hour later when I was on my way to work, the CHECK ENGINE light came on. The next day I had my oil checked in gas station, the young lad put in a quart of oil. My boyfriend checked theoil and said I am low and need some, so we got some and put in and it was fine by the dip stick, the Check Engine light went out, I let my car sit overnight and when I started it, all the smoke that came out of my back exhaust, and then went away after idling the car for a few mins, the CHECK ENGINE light came back on and remains on. If I leave the car sitting overnight, when I first turn it on the smoke comes out from exhaust then goes away but thr Check Engine light remains on, the car sounds fine and runs fine otherwise, what could be causing this?