Is it normal that i'm fat from drinking soda

Not necessarily. From the Mayo Clinic:

"Artificial sweeteners have been the subject of intense scrutiny for decades. Critics of artificial sweeteners say that they cause a variety of health problems, including cancer. That's largely because of studies dating to the 1970s that linked saccharin to bladder cancer in laboratory rats. Because of those studies, saccharin once carried a warning label that it may be hazardous to your health.

But according to the National Cancer Institute and other health agencies, there's no sound scientific evidence that any of the artificial sweeteners approved for use in the U.S. cause cancer or other serious health problems. And numerous research studies confirm that artificial sweeteners are generally safe in limited quantities, even for pregnant women. As a result of the newer studies, the warning label for saccharin was dropped.

Artificial sweeteners are regulated by the Food and Drug Administration (FDA) as food additives. They must be reviewed and approved by the FDA before being made available for sale. In some cases, the FDA declares a substance "generally recognized as safe" (GRAS). These GRAS substances, including highly refined stevia preparations, are deemed by qualified professionals based on scientific data as being safe for their intended use, or they have such a lengthy history of common use in food that they're considered generally safe and don't require FDA approval before sale.

The FDA has also established an acceptable daily intake (ADI) for each artificial sweetener. This is the maximum amount considered safe to consume each day over the course of your lifetime. ADIs are intended to be about 100 times less than the smallest amount that might cause health concerns. "