Fluid filled bump (swelling or cyst) on scalp can occur some times in infants without any specific known reason or minor injury to head while in crib or while handing the baby as such injuries can easily go unnoticed in babies. As the x-ray ruled out skull fracture and as it is not blood, there is nothing much to worry in your child's case. The bump must be localized fluid accumulation between skull bones and scalp which usually will disappear in few months time. The reason for baby being more fussy and erratic sleep may be due to irritation due to the swelling pressing against the crib or due to other reasons.
In any case an Ultrasound would be advisable to know the nature of the swelling and if further necessary an MRI can be done. The fluid can be drained if baby is very uncomfortable, but there is chance that fluid will re-accumulate. If baby is having any other symptoms like not taking food properly, becoming unresponsive, developing fever etc., take him to emergency room consultation. Otherwise, I suggest you need not worry and can take him to Neurosurgeon after securing appointment for further evaluation to rule out any other causes.
Thank you. Hope this helps.