Ok,I found a solution:
I'm using the QGraphicsScene contextMenuEvent instead of the 2 others. Within the event I'm checking whether the mouse is above a QGraphicsItem or not, and I build the corresponding menu. I dont really like this solution as all my functions will be under the QGraphicsScene class and most of them will be concerning the item, not the scene. So it's kinda messy, but it works. Thanks in advance to anyone who has a better solution.