Leafs falling off and turning yellow

[EdwardJerm]

I have a pyracanthas coccinea, I bought it a weak ago, it was fine till today, the only thing that I think might have made this happen is maybe watering it too much, or because yesterday I let it sit in the sun for about an hour... I don't know which caused it, its my first bonsai and I want to figure out what was the problem, it may have also been, that when I put it in the sun, it was outside, and I ussually leave it in front of my window. Please help