Persistent bump on forehead 3 months after Botox - what could this be? (photos)

Good day, I had a botox treatment for forehead lines in November. A few days I noticed a bump on my forehead. I then had a follow-up appointment 2 weeks after the initial botox treatment. The Dr. could not advise what caused the bump and administered another unit of botox in an attempt to reduce the bump. Now, 3 months after the treatment the bump remains (see photo). Please advise on what this could be. I am also going to seek help from a dermatologist. Thanks