Unwanted Touching

Yesterday, my boss kept standing really close behind me and continuously touched my shoulder and I felt very uncomfortable but thought nothing of it . I texted a close friend and she said i was under reacting and that is was weird, my sister also agreed that what he was doing was weird. I even told a coworker today and he told me that it was not the first time our boss had done something like that. What do you guys think? Am i under reacting or is my sister(and friend )over reacting? Btw im 17 and my boss is atleast 45