The fibula is non weight bearing bone (nature has provided it for the muscle attachment) so walking with a boot does not cause stress on the fractured site (saw the x-ray link). The pain you are experiencing can be due to following reasons;
1) Slight improperly fitting boot. This may transmit the weight abnormally and thus the pain.
2) Soft tissue injury which is accompanied with the fractured bone.
3) Both the above concomitantly.
If the pain is moderate to severe, you may need to talk to your doctor or at ER. The other concerning symptoms and signs would be swelling, redness and tenderness, all of which warrant an attention of the doctor. Otherwise you may take an analgesic which has been prescribed (or Advil / Motrin).
Please feel free for your follow up questions.
I would be happy to assist you further, if you need any more information.
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