I noticed a clear bump towards the back of my mouth today it happened within 4 hours it looks like a clear marble what is it?);

Cyst: Would need to see it to be sure, so cannot say definitively. Most likely it is a mucous cyst but various types of cysts exist. They are very common. They can form quickly and are usually not dangerous and will go away or pop open on their own. If it does not resolve in the next few days see your doctor or dentist.

Local care: There may be a small cyst. If it becomes infected, does not heal, then your dentist and physician can take a look. Try to avoid acidic foods, and irritating that area. A mouth rinse such as listerine can be used to prevent infection.