Weirdo with my baby

My mom's best friend, who has always been very good to me, is a bit on the kooky side. She's also a bit of a know it all and oversteps her boundaries. She has done this one thing on three different occasions that really bothers me, but I'm terrible at confrontations. She sticks her finger in my baby's mouth and feels his gums, and says "I can feel his teeth! I think he's teething, that's why he's fussy." Uhhh...no he's 3 months old. Since she and my mom are so close and I see her often I don't want to tick her off, but I also don't want her friggin fingers in my son's mouth! What is a tactful way to respectfully tell her not to do this. 😖😤😩🙄😒