Aunt’s Dog Keeps Running into my Bum

I've known this dog (Broly, gsd) for 7 years and I think we have a good bond. But yesterday if I would be standing or walking, he would come charging at me and run head-first into my bum. No bite, no barking, no scratching. No sound, tail wagging, and repeatedly running into my bum. Not smelling, just tackles. My aunt said he doesn't do that with anyone else. Any idea what that is?