maybe you are just fat?
Very possibly. OTOH,
anything that stimulates the immune system - any disease, any vaccine, any open-wound injury - comes with the risk of immune system over-reaction. I've not heard of what the ironically named
no_hypocrisy described, but it could be that his/her earlier vascular concentrated immune system action in his/her legs, and the booster caused an over-reaction focused there.
The screening questions for any vaccine I've received - Covid, flu, pneumonia - include one asking if the prospective recipient is sick (cold, flu, etc.). I've always assumed this was to protect the nurse administering the vaccine. Perhaps over-stimulating the recipient's immune system is also a concern.