Can penicillin vk give you a sore neck and headaches after finishing the course? both times ive had it for sinus infections ive got a painful neck to move. could this be a side effect for me or just a coincidence. do i avoid penicillin in future?

Coincidence. Almost certainly not due to the penicillin.

Sometimes infections result in fluid and electrolyte shifts affecting cerebrospinal fluid amounts, a process called "meningismus", which would cause both neck pain and headaches. This would not be a true "meningitis", and is benign, self-limited, and reversible. Do not believe, your antibiotic is the cause.