If you sign in to Screencast.com, and then click on the avatar in the upper-right corner of the page it should reveal a "My Account" button. Click that and it will take you to a page which shows you how much storage and bandwidth you've used.
I looked up your account, and it appears that you're using 2.36GB of space, and the limit for a free account is 2GB. It looks like you were paying for Pro (which increases your storage limit) and then canceled. When your account was downgraded to free, you lost the extra storage space which is why you're over the limit.