If you shared folders with others and gave people permission to add content to the folder, that content counts against your storage quota. You may want to check your shared folders to see how much content is stored there. For example, if your folder contains 500 MB of files and after sharing it, other users add another 300 MB of files, then the combined 800 MB counts towards your quota.
You may also have multiple versions of a file, which count against your quota. Check the number of versions you have for your files and see if you can remove some.