Doctor couldn't hear hesrtbest, worried? ?

I am 12 weeks pregnant, I will be 13 on Monday. My Dr tried to hear the heartbeat wth a Doppler but couldn't find it. I saw the baby at about 7 weeks and that was it. I'm so scared. I haven't had any bleeding, a little cramping but nothing unbearable so I figured it was normal. My symptoms completely disappeared after the ultrasound at 7 weeks. I go on Monday for one to see if everything is ok. I'm soo scared and I'm literally in tears. Its my first pregnancy and I just want everything to be ok. But I figure at 12 weeks, she should be able to hear it. She said she heard movement or something but it's not good enough. Is this normal tto not hear the heartbeat? Maybe I'm stressing because I just don't feel pregnant anymore. I've been crying all day. I'm just afraid there will be nothing on the ultrasound. I might just go to the ER tomorrow. Did anyone experience this? I need some encouragement or I'm going to go crazy :(