I have been very very stressed out the past few days, can that cause a lump in the back of my head near my neck?

Stress effects?: You used "very" twice, so it sounds like your stress is extra high. You don't mention a headache. Tension or stress headaches are common. It sounds like muscle tension that could be related to your stress. That is no substitute for a good evaluation by a trained physician. If your symptoms persist, expand, or you are just worried, then see a doctor.