Scary! But I thought I was getting them already like two weeks ago, but knew it was too early - asked my doc about the tightening/strange feeling in my abdomen, he said GAS. Lovely. Can also be caused by dehydration
Remember they don't know everything about pregnancy, and every woman is different. Yes, the HB should be able to be heard by 12 weeks - but some women hear it at 8, a month earlier, because their body is different. You should start showing at 12 weeks, me and another girl in my DDC have bellies already at 9 and 10. Morning sickness should subside by the 2nd trimester, mine is almost gone at 9 weeks. Everyone - and every pregnancy - is different, those are just guidelines
You could just be more in tune with everything, or have a different body shape that allows you to feel it more.
Relax (stress probably won't help), follow the doctor's advice, and everything will be fine

GL sweetie!