My guess is that this actually has nothing to do with smoking/vaping, and it's a weather thing. It happens to be summer and the air is generally closer to your body temperature than when you tried it last. The effect is still tied to the difference in temperature between the air in your lungs and the air outside.
-OR-
I feel like vapor makes my lungs feel heavy sometimes. Perhaps you actually are compressing the air less because you vape.
-OR-
I feel like vapor makes my lungs feel heavy sometimes. Perhaps you actually are compressing the air less because you vape.