I slept wrong on my neck about 3 days ago. A lump formed and it hurts to touch. When I move my neck it feels stiff and it hurts. What should I do?

I have a feeling that the sleeping on the neck and lump aren't related. Regardless the lump needs to be looked at.

I agree that 'sleeping on your neck wrong' is not related to a lump that hurts.
You don't say exactly where on your neck, or the size of the lump, but there are plenty of lymph nodes around the neck and jaw.
Lymph nodes enlarge in response to infection. Common infections might be in teeth or ears, or something like mono or related diseases. So you need a doctor.
This is just an example, not a diagnosis, to get you to go to a clinic.