Why does my chin pull back when I smile? What would help? (photos)

I was thinking about getting chin / lower jowl filler. I do not like how my chin pulls back when I smile. After some reading I wanted to know what my problem was exactly.. Is it ptosis? Would chin filler help or possibly Botox to relax the muscle? I would like to avoid a chin implant if at all possible, at least for now. I like my square-ish chin and don’t want a pointy chin.