Why has my smile changed after masseter botox?

When administered by an experienced surgeon, your smile should not be affected by masseter reduction with Botox.  If your smile has been altered, then it's possible a muscle besides the masseter muscle has been inadvertently injected as treating the masseter alone will not have this effect.  

Botox injections in the masseter muscle can reduce the fullness of the lower face and make it more aesthetically narrow looking.  However, it sounds like the Botox may have diffused too far forward and affected the smile.  I would return to your treating physician for examination.  It is very important to find a board certified dermatologist with experience in these injections for the bet cosmetic results.

There is a possibility that the Botox drifted slightly into your smile muscle. This is very uncommon with this injection. Go see the MD that injected you to re-evaluate your concerns. I have done hundreds of Masseter injections without this problem

The injection of botox or similar products into the masseter is a popular and safe way to contour the lower face making it less square.  In addition,  if you tend to clench or grind teeth,  this can be reduced with masseter injections.   Proper technique is essential as injection too far forward will weaken not only the masseter muscle,  but also one of the muscles used in smiling.   Fortunately, this adverse event is temporary but it can last for  weeks to months if it happens.

Botox acts to temporarily relax muscles of facial animation.  If you have Botox treatments in the masseter muscle, this can affect some muscles that affect your smile.  This effect should be temporary if it is related to the Botox.

Botox injections to the jawline do not allow the same muscles to contract in the same way as before. Sometimes this can alter your smile a bit, depending on where the injections were done. "This answer has been solicited without seeing this patient and cannot
be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."