Painless lump back of neck

I am having the same exact problem...and just recently I am starting to get migraines that hurt so bad, they make me feel nauseous. I too have a lump on the right side of the back of my neck, just below the hairline.

Some lumps are simply "trigger points". You can read about trigger points online - they're points of nerve endings that become inflamed due to injury, stress or overuse. They can trigger muscle spasm and pain - headaches that can feel like migraines because they tend to be one-sided (and painful).

Trigger point treatments like ultrasound (including home ultrasound like Sonic Relief) can help immensely - you simply treat over the area when the pain flares up. It's drug free. Acupuncture works similarly as does EMS or TENS.
Of course, see your doctor too... especially with lumps. But your condition seems very much like trigger points - particularly with it's placement below the hairline. You'll know if you press it for sometime or apply ice and slowly releasing - if you feel relief, you could definitely benefit from home use ultrasound.