Does yawning cause frown lines and wrinkles? I had Botox on my forehead and I want to avoid activities that can affect it

Everyone employs different combinations of facial musculature to create facial expressions.  A definitive way to determine if yawing causes wrinkles/frown lines for you in particular, is to look at yourself in the mirror the next time you frown.  If you are making temporary lines around your eyes, forehead, or glabella, then you will have your answer.  

Any movement that wrinkles your skin will cause lines on your face. If you yawn and your forehead crinkles, you will get lines. The purpose of Botox is to weaken those muscles so they cannot cause the lines.You should keep up with your treatments and just go about enjoying life. Botox will take care of the rest.

Repetitive
movements of facial expression that crease the overlying skin cause wrinkles. Even
if the way you yawn appears to cause dynamic frown lines, it is highly unlikely
that you yawn frequently or long enough to be the sole cause of wrinkles.

lines are apparent on the face by the action of underlying muscles.  The muscles pull in one direction and the lines appear at about a 90 degree angle to the muscle. Facial expression lines occur from frequent and recurring contraction of the underlying muscles.  It is unlikely that one would frown often enough for that to be the cause of wrinkles.  More frequent expressions such as smiling, squinting, and raising ones eyebrows often lead to lines or winkles over time.  Botox prevents the muscles from contracting and, therefore, can help reduce the development of facial wrinkles.