What can cause a 6month old soft spot to bulge with low grade fever?

Depends on situation: If it bulges when baby is upright, it can mean excess pressure has built up in the head & is a reason to go in & have baby checked. Normally hydrated infants can bulge laying down and or during fussy periods.

Recheck in sitting: The anterior fontanelle, so called soft spot, should be felt in sitting position. Feeling a pulsation at soft spot is normal. If soft spot is truly bulging, infant has to be evaluated by primary care doctor.