Three days ago i had a missed shot of meth in my foot and now its swollen and red and is hot to the touch and i cant walk on it .what should i do?

Have it examined. It is likely infected. If so, it can progress deep into the deep layers of the foot & require hospitalization and surgical drainage. Early treatment can avoid that.

Sounds like a cellulitis and needs immediate care. Regarding causation secondary to meth injection, you need critical help. Be open about this, so your doctors can guide you to rehabilitate