Whenever my cat drinks, she rubs her paw against the floor

That sounds so cute lol

they always do that.felines usually need fresh water.they prefer moving water like fountains because of the instinct of not drinking contaminated water.she is digging for fresh water in her imagination

Cats in the wild will bury food that they haven't finished. I.E. a mouse, bird, etc. This is likely what she is trying to do, my cat does it as well.

My cat does it because he thinks stirring his water will make it cleaner. If it doesn't smell clean enough to him he will actually put his foot in and stir it!

It's normal, somewhat common.