Mixed Emotions. Should I be worried or stay calm?

Only 5 and half weeks of pregnancy but tummy big as 2nd trimester. Always feels hungry, can eat double or tripple portions. Possible twins? 1st scanning done and gynae not able to detect a baby foetus. Gynae said, do not be worried, it is only 2mm size. That is the reason why and it is normal. Gynae arranged 2nd ultrasounds to assured baby foetus found. Is this normal?