When im not smiling and just casual the corners of my left side of mouth is regular but my right side is slanted down. do you know why this is?

Likely a palsy : Question. Does the left eye close as firmly as the right ? Can you wink with both eyes equally ? If you open your lips to see all your teeth is it equal both sides ? If not, this suggests a Bell's palsy where a branch of the nerve supplying your facial muscles has become inflamed/functions poorly. You should see your doc for evaluation & treatment.