Stress fracture- The hop test will typically cause pain (hop on the affected leg). A tuning fork will also highlight a stress fracture as the vibration will induce pain if there is a break.

Pain may ease when resting, but flare up during impact/running activities, and does NOT get better as you get warmed up. This can present with swelling or redness in the area.

Shin splints will often hurt more before and after a workout, and can feel slightly better during a run (not always). They will often be bilateral though. Pain is more diffuse and difficult to pinpoint one single spot, as opposed to a general area.

With both of these avoid activities that cause pain. Use an anti-inflammatory medicine such as ibuprofen and apply ice to the area.

Osteomylitis- Bone infection

Cellulitis-
} Both are skin infections.
Erysipelas-

With these 3 try applying mild heat combined with an antibiotic cream/medication.Go see your doctor if you have or develop a fever/temp/shivers/generally become unwell.

Finally it may be a result of a bite :/

Good luck in getting it sorted, keep it dry, clean and if the skin does start to break apply a sterile dressing and I would advise seeing your GP.

Kx