The reason is because your notebook's operating system uses a virtual memory file.
There must always be 10% or more of the Hard disk's total storage capacity available.
If not, what you are experiencing will be the norm. That is not a good thing.
Eventually the operating system on the C: partition will crash and cease to be useable if you do not free up space in the C: partition.
You can o correct the issue by creating a folder on the D: partition.
Start moving the smaller files to the newly created folder and then after verifying they are in the new folder, delete them from the C: user\afbeeldingen folder.
Use Disk cleanup to free more space.
The music on the video at the link is loud, but it does show how to access the Disk Cleanup.
Empty your tmp folder!