Is you heart behind either of the lungs?
Not really, it is between them with some areas of overlap.
Your heart is located between your lungs in the center of your chest, but slightly to the left of the breastbone (sternum).
It is not directly behind them, but rather between them in a position where some parts of the heart are covered - or rather surrounded by - the lungs. By that, I mean that there is a cardiac notch in the left lung where the heart overlaps.
I hope this helps, and let me know if you need anything adding:)