The 90% is a health rating based on the SSD's remaining life. However it looks a little bit low to me based on how much you've written. For a Crucial disk you'll get a more accurate reading from Crucial Storage Executive. You've done a lot of writes in a fairly short space of time, if your not running any I/O intensive workloads then I suggest you leave the Task Manager open but on the details tab add the column 'I/O bytes written'. Alternatively you can use Process Explorer and that will tell you how much each process is writing, I would just leave it open all day.
Recently I had an issue with my machine writing up to 1TB a day, it wrote over 80TB to my main drive in a short space of time. After investigating I discovered it was Kaspersky Antivirus background drive scans doing it, unfortunately it was by design and unfixable. I looked at a few anti virus programs and found that it's quite common for them to write large amounts of data to the OS drive when doing a full scan of a big drive. The only one that didn't was Norton so I've switched to that, probably not you issue but worth a check if your not intentionally running heavy workloads.