13 weeks pregnant and worried?

Hi Everyone, I am almost 14 weeks pregnant, the last time I heard my babies heart beat was at 12 1/2 weeks, the last few weeks I have had severe back pain, which I hear is very normal. Out of no-where the back pain has stopped overnight, I am very worried that something is wrong with the baby. I went through a miscarriage last year and I can't forget how horrible it was. I am scared I will miscarriage again. I am considering renting a doppler, My doctor says not too, because it will drive me crazy if it doesn't work, and I am not 100% sure where the baby is placed. Should I worry that my back pain has suddenly stopped over night, and should I rent a doppler from a medical shope???