Thank you for your query at DoctorSpring.com
I understand your concern.
From the picture that you shared, it is evident that you have a tiny little bruise on your back. These are common with such bites and it does not look anything to be concerned about. Only in certain cases, where the skin comes off, and it bleeds, there are chances of the area getting infected.
In your case there is no such concern and you should just leave it alone for spontaneous recovery.