Toddler Hitting Head.... Help!

So my 15 month old has been slamming her head on the ground for the past week or two anytime she’s upset... My mom said it’s normal because I did it but I’m just not sure ... Today she slammed her head so hard it left a big purple bruise so I’m getting kinda worried about this behavior. Anybody else deal with this and know how to make it stop ? I don’t understand her hurting herself like that on purpose 😕