Is it normal for dental filling to fall out within one year?

A year is definitely waaaaaay too soon. Talk to your dentist, honestly I think she owes you a free repair, or at least a big discount.

But you do have to get this fixed immediately, or you'll be looking at a root canal in a short order. The inner part of the tooth, that is all open right now, is soft and fragile - nothing like the outside enamel. Things head south very quickly unless you get it patched up.

But don't yell at your dentist, you don't know what happened, and if you're mean right off the bat, she just might cut you loose.