Should I be worried ..

I went to my first appointment three and a half weeks ago . I was seven weeks exactly . The doctor pointed out the heart beat and said everything looked great .. But I could barely see the heart beating . I was a little further along at my first US with my son and I believe I could see it better ! I've just been worried that the heart was beating slow . My doctor didn't sound concerned about anything and said everything was healthy ... But still maybe I just worry to much . This was my seven week US​