Maybe JUSTIN BIEBER is just acting out post-breakup . . . but a series of interesting pictures of him hit the web yesterday.

--First, there's a shot of him at a meet and greet, posing with a female fan . . . and CUPPING HER BREAST.

--He was also photographed yesterday flashing his chest and underwear, before later slipping into something more . . . weird.

--That outfit featured what appeared to be GIANT TODDLER PANTS, complete with extra diaper room at the hips . . . and multicolored tennis shoes with yellow WINGS affixed through the laces.  (--Here are some pictures.)  (Oh No They Didn't, Daily Mail)