Why is my british bulldog scared of some things and jumpy?

i have a 2year old bulldog which i bought in august this year and she just turned 2 in august this year.on her certificates and notes tells us she has already had 2 homes before she has came to us and it really worries me if anything has happened to her in her past homes the sound of a carrier bag scares her also if i drop anything on the floor she's really jumpy and scared and runs off.even certain noises from outside.i was just wondering if anyone can help me if this is common in some bulldogs or has it been her past homes and is there anything i can do to stop this or help her.thank you.