Up or down a notch?

(I apologize for the silly question ahead)

I've lost some weight recently, and I was able, for the first time today, to close my belt buckle using a notch higher than usual...

For the life of me I can't figure out if I "went up a notch" or "down a notch"?

Is this phrase even applicable for an actual belt?

down a notch means down a level. It is used in sentences like

Turn the volume down a notch.

or, figuratively, in sentences like

Anyone who names a board game after himself needs to be taken down a notch or two.
Soon he was taken down a notch.

The expression you are looking for is probably to notch (something) tighter.

She notched her belt tighter.

Normally, waistbands are described as being 'taken in' or 'let out'. I would suggest that when describing notches on a belt (notches in a belt?) that the proper phrases would be something like, "After dieting for two weeks I'm buckling my belt in one notch in."