My 15 Weeks Old Kitten Has A Hanging Tummy, Like An Older Cat, Also He Has A Little Lump On This Tummy, It's As Big As The Tip Of My Finger Round And A Bit Hard. What Could This Be?

Most of the diseases listed above are very uncommon in a kitten of this age.  Hernias are very common in young animals--both dogs and cats.  It represents a developmental problem where the body wall does not close normally so in most cases fat from the falciform ligament becomes entrapped.  Some hernias are reducible (you can push it back into the abdomen) while others are not reducible.  It is less common but it may be that more of the line alba (connective tissue that connects the muscles of the abdomen) is not formed and the large hanging area is abdominal contents. 
If your kitten is not eating, drinking, acting lethargic, or behaving unusually it needs to be examined immediately.

Your kitten could have an umbilical hernia - where the abdominal muscles allow the intestines to protrude. It's not always terribly dangerous, but sometimes if it's not taken care of by surgery, that part of the intestine can become necrotic (dead) and cause a lot more serious problems.

Hanging tummy is not a problem if your cat is eating, drinking and playing. Some cats have this type of tummy. Now coming to lump in your cat. There can be many possibilities and final diagnosis will be made by your vet.
This lump can be reaction of injection, abscess, hernia, cancer, tumor, cyst or trauma. I will give you possibilities of tumors and cysts.

• It can be fatty tumor (common)
  
• Cutaneous horns (Like wart)
• Dermoid cyst (Rare)
  
• Basal cell tumor (common)
  
• Sebaceous cyst (common)
  
• Lymphoma

Now visit to vet for the diagnosis and treatment.