Thermoclines don't have to be just a single small zone of water. Though there are several different ways to define a thermocline, many often referring to just the zone of greatest temp change, but by technical definition, the water (zone) dropping at least 1 deg. C per meter of depth is a requirement. On larger thermoclines, you can have an upper and lower level to that band of falling water temps ('top' and 'bottom' of the thermocline). So you can have a large band of water that meets that technical definition, especially early in the year, like now, when stratification hasn't set up well, yet have certain depth levels within that larger area where you have more dramatic drops than the rest of the overall zone. The chart below from a local reservoir demonstrates this well. The area highlighted by the red band represents the thermocline by the technical definition, yet you can see two smaller depth ranges where there is a significantly greater drop in temp over a shorter range (gray boxes) which might be enough to show on a graph similar to what you posted.
That said, and as has been mentioned by others, there are other possibilities that could explain what you saw on your graph depending on exact weather and water anomalies in your area. One thing not mentioned yet since you mentioned "night," is the bottom band could simply be zooplankton or similar making a migration upward in the water column due to a low light situation (referred to as a "diel migration"). This is very common in summer on some of the reservoirs I've fished.
So short answer - maybe, maybe not