Thanks for posting on HCM
This could be a result of an allergy
( due to contact with an unknown object) reason why it just went off on its own, could be as mild bacterial (staph) skin infection
) which will require consultation for confirmation and appropriate treatment ( antiobiotics and antiinflammatory).
As per your presentation, I do not feel you will require any further consultation as situation seems to have resolved itself but it will be good if a doctor could take a look at it so you can know what it was and if possibility of recurrence in the near or far future (actually can not be predictate by the doctor).
Hope this helped