2002 Mercury Sable problem???

My stepmom has a Sable that just went in for a check up at a car shop where my stepmom knows the owner. They didnt mess with the engine or anything, all they did was change the oil and check some other stuff. She picked it up from there and drove it home which is about a 45 minute drive. The car was perfectly fine all the way home. When she got home she decided to fill her tank up because the gas light was on. *She went to BP to get gas. But yea, according to her, the next morning she said it started to shake during idle and when she accelerated. Later that night which was last night...i took it around the block to see what she was talking about. From me driving it, i realized that while you're are stop and at idle it shakes a little every once in a while. 800rpm-1800 rpm it shakes and then it stops. We think its water in the fuel line. We dont think its any transmission or engine problems. All spark plugs are firing. I checked them all.