Lump on left masseter muscle after Botox injection? (Photo)

I would ice the area and maybe take an antihistamine. however you would need to go back to your injector to assess this closely. Good luck. 

It might be a newly developing hematoma or more rarely an abccess formation. I would consult with your injector to reassess and act accordingly. 

Its hard to tell, but you should definitely follow up with your treating doc ASAP.Good luck from NYC

Generally after you have your first injection of botox into the masseter muscle, you may notice a lump when clenching a few days later for a few reasons and this is very normal. 
1. The botox hasn't completely relaxed the muscle. Remember it can take up to 14 days. 
2. The masseter muscle was so large that the initial injection does not completely relax it and after 14 days if this lump persists, that portion of the muscle will need an additional injection in order to make it relax. 
Whenever I inject clients in their masseter muscles, I'm always sure to have them come back in for a post-op appointment 2 weeks after. 

Thank you for your question yuke14. I understand
your concern. Botox is a purified protein used to address wrinkles associated
with facial expression. Botox can be placed in the masseter muscle in the lower face to address pain associated with TMJ. This also works to contour the lower face. It is difficult to make an assessment without before photographs. However with any injection there is a risk of a bruise or hematoma, which is a collection of blood in the soft tissue. Hematomas resolve slowly over time. Some respond to IPL or other vascular lasers, cold compresses, and possible topical or oral arnica. Please consult with a doctor for specific
recommendations. Good luck!