What could be the cause of acne after Botox?

It's unlikely that the Botox itself caused you to breakout with acne. The breakouts maybe related to stress or certain cleansers or lotions using before or after the treatment. I would discuss the acne with your treating physician.

Sometimes the stress of the injections itself can make one's face break out. This is simply how your body works and it may happen again if you have botox or it may not, now that you're familiar with the procedure. Good luck from New York City

Sometimes the sheer injection can cause trauma or irritate the skin. Other things that could predispose to acne/ dermatitis/ skin reaction could be the cleanser or gauze used to clean the skin. You should made sure that everything is done under clean/ sterile condition to minimize any secondary infections. We typically use EtOH swabs and all our injectors/ including myself use soft non woven sponges and gloves.