YOur concern is suggestive of torticollis which in simple terms mean sprain on the neck.Treatment for torticollis is targeted to relax the contracted neck muscles involved. Treatments include medication, physical devices, , physical therapy, stretching exercises, and surgery. In most people, torticollis resolves in several days to a few weeks. A few people will develop continuing neck problems for months to years. Persistent neck muscle spasms may require referral to a neurologist or surgeon. I cannot say this is due to swimming but awkward positioning while sleeping could be a reason. I would give local heat and gentle massage and NSAID drugs like Ibuprofen if pain is very severe. Take care and regards.