Check your Levels & Project North settings under the Project Preferences. My guess is your Project Zero is set to +100'. For future projects, model from 0'-0" as your model's base elevation, and then control the referenced elevations via the Reference levels.
Scott Graham, AIA LEED BD+C
Principal | BIM Manager
Muhlenberg Greene Architects, Ltd.
Wyomissing, PA 19610
V6.5-26.0