Measure small

I had my first visit today at the doctors.. And the app I used to get pregnant says I'm 8 weeks and the did the ultrasound and it says the baby measures at 5weeks and heart rate is low.. I haven't had any bleeding or bad cramping... She says I have 80% to have a healthy baby.. They took blood today and I have to more visit to go too to see if my levels will grow.. I am preying and won't stress.. But lately this heart burn is killing me?!