Laureen, It was a good call to turn back as a precautionary measure. It could be a number of things. Did you recently fill up with gas? It may the fuel. Or it may be coolant seeping in the head thus mixing with fuel. If this were the case, the RAV4 would be running very rough. When you checked the oil, did you notice any frothing or bubbling on the dip stick? At this time, is it running smoothly or roughly at idle? Is there any moisture coming out of the tailpipe when it is running now? It may have been none of the above and it could have been something as off the wall as water in your tailpipe and or muffler from driving in deep water from heavy rain.