Is the lump on your skull or where your skull meets the cervical vertebra?

It is not uncommon to injure the joint between the skull and C1 cervical vertebra. This area can become inflamed causing symptoms to worsen or come and go as the inflammation goes up and down.

A physiatrist (Physical Medicine and Rehabilitation specialist) may be worth finding.

You can also ice it regularly to see if it goes down.

My best to you.