I recall a similiar experience a few years back.
Overcast at around 5,000 feet, otherwise clear air, no local thunder storm activity, but maybe a cell around thirty miles to the south.
Level, low ground with a small ridge to the north up to 500 feet. Light wind.
At 4,500 feet I got caught in presumably a micro burst. With the engine at full chat I was getting a 1,000 fpm descent in totally smooth air. Fortunately there was a grass strip I know over my left shoulder, but at 500 feet and still descending, enountered light to moderate turbulence and air rising as quickly.
Never quite understood the events of that day or the circumstances that caused the micro burst. I always wondered at what height the descent would have stopped if I had not passed into the rising air.
I will also remember I thought for some while the problem was with the engine and couldnt understand why in spite of everything appearing normal I was descending at an alarming rate. Of course everything was normal! (at least so far as the engine was concerned).