There is no reasonable healthy explanation for your grandmother touching you that way. The fact that she would wait until the two of you were alone suggests that she knew it was wrong. She crossed the line when she should have been one of the adults who was keeping you safe.
I’m not sure what you mean by asking if the incidents “count” as molestation. It happened. It shouldn’t have. But then things get complicated. People are rarely all good or all bad. Although it would be easier to just judge your grandmother, there are probably layers to this story that need to be explored — especially since you still live with her.
Rather than fret about whether something is or is not molestation, you and your therapist might find it more useful to talk about whether it would be helpful to you to talk to your grandmother about that time in your life and how she thinks and feels about it now. It might comfort you and help you get past it if she were to offer you some information and, especially, if she were to apologize in a way that shows that she really means it. If, on the other hand, she would deny it or blame or shame you, you don’t need to put yourself through that. Do talk it through with your therapist.
I wish you well,