Lump on back of neck rash like bruise?

Uncertain. This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes. Once accurately assessed a treatment plan can be developed.
Insect bite? Can be from a spider bite but there are many other potential answers. Does the lump hurt or itch? Is it getting larger?
Impossible to say. It is impossible to give much specific advice without being able to see the area involved. Have a doctor take a look.

Related Questions