Brief Answer:
possible causes explained below...

Detailed Answer:
Hello and Welcome to 'Ask A Doctor' service...
I've gone through your query in details...
I can understand your concern...

Well, as per your description, this could be an impending skin and soft tissue infection (bacterial most likely) i.e. some infected cyst, or even an abscess.

Since you've been treated already, it could've been re-infected (a secondary complication of the same or another bug). The callous could be due to deposited secretion and debris. I won't suggest putting the band-aid over it at all.
Please don't put Silvadene on it either.

Just clean the area gently with lukewarm saline water with sterile gauzes.

After that soak another gauze piece with Betadine (Povidone Iodine 10%) topical solution and apply over it.

Change the dressing only if gets soiled, otherwise change it once daily...Avoid putting any heavy pressure on the big toe, try to keep it elevated for the time being.

Give the fact that you're diabetic, and feverish as well, seems a definite need for further antibiotics therapy. However, since it was already treated before, there's a possibility of resistant bugs (Penicillin group, so Amoxycillin may not be useful for you in this scenario).

I'd recommend a wound swab gram stain and culture sensitivity analysis (antibiogram) to decide on the further course of antibiotics.

Please discuss this with your doctor tomorrow and proceed further.

Let me know if I can assist you further.

Take Care
Kind Regards

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