Post lower blepharoplasty bag when I smile. Is this skin or muscle? (Photo)

Thank you for sharing your case and photos.  You have had the right amount of skin removed with surgery.  The bunching you see is from the underlying orbicularis oculi muscle. Some residual muscle is necessary to help you close your eyes and blink.  There are several options to treat this including surgery or Botox.  Consider a consultation with a board certified oculoplastic surgeon to discuss your options.  Good luck!

There are a few issues here. At rest you don't appear to have redundant skin, so there is probably not much excess that could be removed or you might get an ectropion.  It looks like you had fat removed, so you are volume deficient.  This increases the amount of wrinkling when you smile.  Adding filler to the lower lid may help.  You could also do some laser resurfacing to try to improve the skin texture. Botox to the orbicularis will help soften the muscle roll you see under the lashes when  you smile. When people have a very large muscle here, a small amount of it can sometimes be removed.  If too much is taken it can affect your lid tone and blinking.    

You can tailor a muscle roll, reduce lower eyelid skin laxity and move up the we cheek junction with a superficial cheek lift.

It appears that you may be looking at your orbicularis oculi muscle, though pictures of you looking up would be helpful to see if it is fat, or bags.

from your photo it appears you have had the maximum removal of skin as judge by your resting eyelid position.  There is always crinkling of the skin with smiling but you do appear to have more of a bag that is probably due to the muscle.  You could have your surgeon give you a trial of Botox injected into the muscle to see if you get improvement.  If you do then you could continue with botox if that is something you desire or have the muscle surgically debulked.  The surgery is relatively straight forward.  I hope this helps and best of luck.