Why am I out of submission storage?

I recieved an email saying my account has been downgraded and that I am currently out of submission storage. Why is this?

The free form allows up to 100 submissions and I am only currently at 28. 

Hi,

Thank you for contacting us.

You have reached your form submission limit which was 500. After account downgradation, your submissions from your previous subscription will be kept in your account. 

Please delete some of your submissions. I would recommend you to download your submissions before deletion. Have a look at the user guides below for further information.

https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data 

https://www.jotform.com/help/73-How-to-Download-Form-Submissions-as-Excel-CSV-PDF

If you need further guidance, please let us know.