Hard immovable lump on back of neck near hairline (left side). Appeared last week and seems to change slightly in size. Should I worry?

Yes, possibly! You should see a primary care physician soon for a thorough evaluation. Your "lump" can be something common that doesn't necessarily need treatment (lipoma, sebaceous cyst) or something treatable (boil, abscess) or something potentially serious (enlarged lymph node) or something definitely serious (cancerous tumor). Since you are in Wash, DC, you can make appt to see me.