My baby was born with a head circumference at the 5th percentile, and now at 11 months its at the 95th percentile. is this rapid growth considered normal or is it worth looking into?

I would likely do some form of imaging to look into the inside of the skull. Sometimes they can do a simple ultrasound through the soft spot if it is still present. If not a simple CT of the head would do. You did not mention any of the other body parameters, like length & weight. If everything is in the same range other issues would need to be investigated.

I agree with Dr. Ferguson. Would want to rule out hydrocephalus or a buildup of cerebrospinal fluid in the brain. You also didn't mention if his or her developmental milestones are being met. That would also affect her into the workup