Can’t tell you “why”
Does “numb” mean no sensation at all or something else? Numbness of an entire hemicranium w/o other Sx is unusual.
(Doubt the “soreness” is related.) In the brain, ?migraines or ?MS, much less likely stroke or tumor. In the neck, maybe C1-C2 cord lesion (?MS ?tumor) but unlikely. It’s completely unrealistic of you to expect an answer to “why” here. See your doctor, sooner rather than later.