It is not normal or appropriate for an adult to pretend to be nursing a 7-year-old. The exception would be if your grandmother was suffering from dementia. If that is the case, her actions were due to the dementia, not an act of abuse. Either way, though, it may have affected you.
However, although you were being victimized, it doesn’t necessarily mean you were traumatized. Different people react to victimization in different ways.
By all means, talk to your therapist about it. I can’t determine on the basis of a short letter whether your reaction to it is a sign that you are covering up feelings about it — or if you really did write it off as something weird your grandmother did and moved on. Either is possible.
I’m glad you are already seeing a therapist because you have someone who knows you well with whom to talk this out. Please don’t avoid it out of embarrassment. You didn’t do anything wrong. Your therapist won’t judge you but will help you explore whether that event is having negative impact on your life now.
I’m very glad you wrote. It might help you raise the issue with your therapist if you just bring your letter and this response to your next session.
I wish you well.