If my scalp & skin was itchy after lumbar puncture, which is that- an allergy or a side effect of the lidocaine injected in my back?

Hard to know: And it could of course be unrelated or be a result of whatever it was that prompted an LP. However, if you had rash/itching on your scalp, it is more likely to be some kind of allergy rather than just a local reaction. So be careful and remember to tell the dentist or doctor next time you need a local anesthetic. They could use another one besides lidocaine. Good luck!