The most common cause of this type of problem is an engine that is low on oil. 1qt low on oil will allow the oil to slosh enough to starve the engine of ooil pressure. Check and adjust the oil level first.
The second common problem is an oil pressure sending unit thats failing. The sending unit it the piece on the engine that reports the oil pressure.
I would look at the oil level before anything else..
I checked underneath the vehicle. It seems to be leaking from vehicle about mid-chassis. Any thoughts?
It could be leaking off of the rear end of the engine. Possibly the oil drain plug gasket is no good. Thats the best option.
There are other options such as the rear main oil seal or back of the valve cover. I would have it looked at on a lift for the definate answer to where its coming from. Until then, just monitor the oil level. The leak may be very slight but just enough that over time the level dropped enough to turn on the warning indicator...