What causes Botox injection to cause severe burning and pain?

I’m sorry to hear that
it was so uncomfortable. I do not know what needles your physician used, but
generally, it is a very fine needle that is used to administer Botox. It is
possible that it was a different needle type that you were not used to.
Regarding the Botox used, I cannot speak to the product that your physician
used, but most physicians will obtain their Botox directly from Allergan, the
pharmaceutical company that manufactures Botox. It usually takes 10-14 days to
see the full effect of the Botox.

The most likely possibility, since you've been treated with Botox several times previously without issues, is that the Botox was reconstituted differently during this injection.  Preserved saline tends to burn less than non-preserved saline - however the cosmetic results should be the same.  Second possibility is that the needle used was slightly larger than your previous injections.  Another less likely possibility relates to your cycle.  Some women do report having a much lower threshold for pain at certain times, resulting in more discomfort during Botox injections. You'll begin to see effect in 1-2 days, and full effect in about a week.  

Similar to your previous experience, Botox injections should not produce any significant or long-lasting burning sensation.  A meaningful amount of baseline anxiety can magnify the perception of discomfort.

There are times when your body is more sensitive to injections, especially in times of stress.  The other cause for the pain is that some other dilutant other than sterile salt water was used.  I would contact the office where you had the injections and inquire with them.

When Botox was approved the instructions were to mix it with saline. Physicians that use this often quickly learned that using bacteriostatic saline removed the sting from  the injections. The portion of the saline that makes it bacteriostatic also keeps it from stinging. If your Botox was not mixed with this it probably did sting.