Welcome To Healthcaremagic forum,
I would like to know few things here:
1) what is your age.
2) what is your normal skin color.
3) do you have thyroid
With the history you have provided it seems to be a post inflammatory
de-pigmentation or pre-vitiligo
Both condition can occur after a mild trauma.
I would advise you to wait and watch for few weeks initially.
If the patch is spreading or is becoming more prominent ,then I would suggest you a treatment.This would mean that it is vitiligo.
If the patch remains as it is or regresses,then it is a post inflammatory de-pigmentation.
This is just for diagnostic purpose,since your treatment would remain same in both the case.
You would require a mild steroid
cream or tacrolimus cream for local application.
If the patch is spreading fast then you would require oral steroid and other immunomodulator.
You can also go for phototherapy(PUVA
& NBUVB) depending upon your age.
Hope this answers your query,if you have any further questions ,please write back to us.