What is your source media, your project settings, and your output settings?
Something doesn't match.
The answers will be in the details in order to resolve your issue.
What version of Premiere Elements and on what computer operating system is it running?
What is the brand/model/settings for the camera that recorded your source video for your project? This is my quick way to get at the properties of your source files rather than have you download and install a video audio properties readout program.
Did you (manually) or the project (automatically) set the project preset (project settings). They are supposed to match the properties of the source media.
If you do not know the project settings, then please go to the project's Edit Menu/Project Settings/General and tell us what is listed for Editing Mode, Timebase, Frame Size, and Pixel Aspect Ratio (even if the fields look grayed out).
What did you select in the Publish+Share/ area of the program for your export (details please)? Example...did you select
Publish+Share/Computer/AVCHD with Presets field = MP4 H.264 1920 x 1080p30 or something else?
And for whatever you selected (Publish+Share/Computer/something else, did you use the default settings for the preset that you selected in Presets?
Or did you click on the Advanced Button/Video Tab there to customize the export settings?
What computer player are you using for the playback of your Premiere Elements export?
Let us start here. As has already been pointed out "Something does not match". I intend that the details given above by me will give you specific direction as to what is needed to help you resolve your issue. Any questions or need clarification, please do not hesitate to ask.