Lump on back of neck rash like bruise ?

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.

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?

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.