So scared. :'(

I had an ultrasound today at 12 weeks 4 days. Everything is fine except the heartbeat. Baby measured 13 weeks 1 day but the heartbeat is 112bpm. The doctor seemed very concerned. Put me on bed rest, told me to drink 20 oz an hour and eat every two hours. Has your baby's heartbeat ever been this low and everything was okay? Please anything helps.