almost 7 weeks pregnant and spotting put on prometrium 200 orally

I am scared and would like to know if anyone has had this same situaton? I started spotting on friday and i went to the e.r with my hubby yesterday after being poked more than enough times They performed exams and told me the baby was fine and the heartbeat was 114 bpm... They discharged me and said i should take it easy and rest. They called this a "Threatenend miscarraige" This is my third pregnancy and i never experienced this before.As we are leaving the hospital .My prenatal Doctor calls stating my blood work came back and my hormone level dropped one. They decided to put me on Prometrium 200 orally because my placenta not making enough progeterone. im supposed to be on these pills to at least after 12 weeks.. This is to help my pregnancy stay strong. how long does it take to work? and does it really help? do they put you on bed rest due to this ? Im just worried.I pray and hope this helps. I took my first pill today and it put me out.so i will be taking them before bedtime from now onn.....Thank You all in advance for any comments....