Flowers, Bees, Prunus...

I have this little Prunus (some kind of ornamental Prunus species, maybe incisa or serrulata but I'm not sure) that I'm trying to growing in a broom style. Because it's flowering so nice I've brought it up on our balcony. This morning I have noticed a big bumblebee flying around and landing on this tree.


Two years ago I have rooted a little cutting from this tree. It's really tiny, about 10cm high, yet it's flowering.
It was overwintered in an unheated room and was the first tree to bloom this year, in the middle of march.