Let your injector know; however, there is no contraindication to have both injections performed on the same day. While both injections can cause "flu-like" symptoms or headaches, there is not much you'll do differently if the source is the Botox or the source is the flu shot. I wish you well!
There is no direct contra-indication to having both Botox and flu shot done on the same day. If you had Botox before and know how you react to it, then there should be no problem with having both on the same day.