Stranger danger!

My son (12w) started crying when a stranger holds him today! My stepfather came to visit and when he held him he started SCREAMING. As soon as I took him back he stopped crying. We tried again a little later maybe hoping it could’ve been something else but he did it again. This time, my mom grabbed him (she comes everyday) and again he stopped crying. I was a little shocked because before then he did well with strangers. Also, I read that this was supposed to start around 6months. Anyone have similar stories?