What Causes Strong Heart Beat Of Fetus During 32 Weeks Of Pregnancy?

32 weeks pregnant and doctor called and told me to call him when I did he said he didn't have my chart and didn't know why he called what could be wrong? had growth sonogram last week and tech said all looked good and when I saw the doctor after the sonogram he said heartbeat strong and would call if they saw anything wrong but didn't expect to. I have only gained 11 lbs so far