I had front teeth extracted and a bone graft for implant about 7 days ago. I have flipper but it is not comfortable.

There are many temporary options, the best one is a Snap-On Smile, you’ll be able to chew and it’s strong to last during your healing period. There are different brands, but the Snap-On smile from the same company as Lumineers is amazing. Other options can be an Essex appliance, bonded composite pontics, or activate your implant sooner.

Hi!
Don`t feel bad, this is just all part of the process. Partial dentures are not quite comfortable, specially not just 7 days after you got surgery since you are still swollen. It will fit much better when inflammation subsides. Just remember that everyday it goes by, it will be even better. 
-Dr. Meza-

I would go back to the dentist who made the flipper and get and adjustment. As a general dentist I usually make the flipper for a patient who is going to have a tooth extracted, and deliver it after the extraction. Often times I have to relieve the bite on the flipper so that all the teeth touch as they normally should when you bite down.
Likewise, I find that the lab sometimes will make a flipper with a thick ledge on the palatal aspect that can be uncomfortable for the tongue and get in the way when you speak. Though it may be tough to talk with a flipper in at first anyway, I often find that smoothing out this ledge and thinning the flipper slightly in the area adjacent to where the tip of your tongue is helps make it more comfortable and natural for you.  Hope this helps!

Flippers are never the most comfortable thing you can have in your mouth. They are never that stable, strong or capable of providing any form of function (eating). but they should be adjusted so your back teeth do remain in contact during the healing phase.  You should return to your doctor to get an adjustment to ensure your back teeth can come into contact.   
Another option that may be available, and this depends on your situation, is bonding a temporary tooth in the gap. The bond would be to the teeth on either side of the gap so those teeth would have to be healthy enough to bond too ( If there are crowns on the adjacent teeth the bonding won't be an option) 
A bonded temporary tooth can be a nice aesthetic option that is more comfortable than a flipper but can only be used in certain situations and like the flipper can't be used for biting on. It is meant as a temporary solution and has to be treated very carefully while you are waiting on the Implant treatment.