Normally when Windows is given the automatic choice of page file size it will keep the page file on the (C:) or system drive.
However if the system drive is getting full with less than 20% free space and another drive is available, Windows will use that other drive.

Check the used against free space in the system drive and if less than 20% free space, look towards removing unwanted stuff, and shifting some more of the wanted to a different drive if possible.
Otherwise it means installing a bigger system drive.

You shouldn't really allow any system drive to become that full that there is less than 20% free space, it can eventually lead to malfunction of the operating system and even finish up with a damaged drive.