B737 Captain reported encountering wake turbulence at FL350 in ZDV airspace that resulted in minor injuries to a flight attendant.

Narrative

We were just leveling off at fl350; FO asked if it was ok to let pax (Passengers) up and I said yes. Ride had been smooth in climb and no adverse reports. Just after turning off the sign we got a jolt of wake turbulence. No upset and autopilot stayed on. FA called and asked about ride status and asked to keep the seatbelt sign on until clear of any further turbulence. We accommodated her request despite no turbulence reported. FA later called about an hour later to report she had a sore neck and headache as she was just unbuckling her seatbelt as we hit the wake turbulence and would likely file an injury report. I asked if she was fit to continue and she said yes. After landing she reported same symptoms. She did not contact medical advisory service during the flight. No diagnosis of injuries only neck pain and headaches.