More info needed. Let's start with exactly what version of Premiere you have, as shown in the About box. [Please include this data point with all questions.]
Also, please provide information about the source content, the sequence settings, and the export settings.
- height and width (in pixels)
- pixel aspect ratio
- codec of the source content
- preview file format for the sequence
- Export format and codec.
- Export bitrate
Posting screenshots of the sequence settings and export settings dialogs is a quick way to convey much of this info.