She knew you were touching it, she knew you were not groping it. That may well have been why she was so permissive.

She may have been angling for more, she may just have been a close friend who wanted a back rub (let's be honest, back rubs are fucking awesome), but you made it clear that she could stop you at any time and she not only failed to do so, she clearly stated afterwards that she was good.

I think you're quite fine. If you're interested, they're might be more there. If not, you've got a close friend. Either way it might be good to check in with her about how she feels, but I don't think you're in trouble for accidentally massaging a bit of boob.