Little fine bumps on face. has happened before but it clears up in few days. what could cause this & what is this ?

Comedones: What you are describing sounds a lot like mild acne. Things that come and go on the face which appear as fine bumps are probably the first forms of acne called comedones. These can resolve on their own, but usually come back. There are many treatments for this type of acne and if you want to prevent it from forming it would be a good idea to see a dermatologist.