How likely can my infection of hepatitis a become fulminant hepatitis?

Unlikely: Hepatitis a does cause some inflammation of the liver, but unlike hep b or c, does not cause ongoing chronic inflammation ("hepatitis"). It is a self-limiting disease that in the short term can make you sick, but doesn't have any long-term sequelae.

Not very: Likely. I would venture that less than 10% of all HAV infections take a fulminant course.