No doubt, it was a windshear. Windshear is not necessarily to be in turbulent air. I believe it was the kind of windshear, which may be induced due to inversion. The inversion may prevent two different air masses from mixing each other. These two layers of air masses may have completely different wind speed and directions.
In your example, most likely you were getting stronger and stronger headwind as you were climbing. This wind speed change may be as much as 8 knots per each 100-foot climbing. So due to inertia of the airplane mass, the apparent (not actual) performance of the airplane increased for some seconds. However just after crossing inversion layer, the headwind ceased or -maybe even worse- turned out to be a tailwind. Again, due to inertia, the apparent performance of the airplane degraded.
I believe that's all.
Jabbara