Probably normal although it's hard to say from your description. Start with a full charge - look at the voltage. Watch the battery info screen intermittently throughout the flight. The C2 button is usually programmed to do that. Initially everything will be in the green. Fly around to the 50% level or so. Flip over to the screen again. Voltages drop. Typically in concert with only a small amount of variation. Drop down further and you will see one or two cells start dropping into the yellow range. THIS IS NORMAL. You will definitely notice it if you load the UAV (push up on the throttle). As you deplete the battery all four cells will be yellow. Go past 30% - you will start getting the low battery warning.
If you want to be obsessive compulsive about it, you can download your telemetry to Healthy Drones and you will see in glorious detail how the batteries react.
So, I'm going to bet you are OK. Carefully RTFM several times and understand what everything means. It is REALLY a good idea to flip up the battery screen every once in a while while flying.
