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. 

which narrows the cause of the pain to the diluent used...most doctors basically use sterile salt water with some material to prevent bacterial from growing in the vial...when doctors stray from this, then the pain factor may significantly increase...then there's the stress level...if you were under more stress than normal, then even the same treatment may be unusually painful...

It is possible to get severe pain and burning from your Botox injection if the injector diluted the bottle with sodium bicarbonate instead of the usual saline.  It also would decrease the efficacy and duration.  This may occur because in a busy office the medical assistants may be mixing/preparing the Botox for their physician and they may accidentally grab the bicarb. bottle instead of the saline.  Unfortunately, the bottles do look the same.  I have been injecting Botox for over ten years and have seen and heard of this mistake before, though it is rare.  It is not harmful, just makes the experience unpleasant.  Another way to get pain during injection would be if a nerve was injected.

Typically Botox injections should not be very painful...maybe minimal transient burn effect upon injection that immediately ceases after injection.

Sometimes you could have burning if the injector is not using Bacteriostatic Sodium Chloride.  You could check this with your injector.  This is very difficult to "diagnose" as pain is so subjective.

Botox injections (neurotoxin) involve multiple, very small needle injections that should feel like a small pinch or mosquito bite. To diminish the pinch, a topical lidocaine anesthetic cream and/or ice may be used. Some days patients feel more discomfort than on other days and sometimes one side is worse/better than the other side. Why? Because pain is very subjective, difficult to quantify, and very individualized. Botox, Dysport, and Xeomine are neurotoxins used to diminish facial muscle contractions causing wrinkles: the 11 lines, forehead horizontal lines, crows feet., etc. In my experience, none of these neurotoxins are known to be more painful. Dysport usually starts to work in one day while Botox takes three days.