Babies heart beat dropping...

Last night I went to the er because my lower back was hurting so bad I was throwing up. The doc said my back pain is being caused by gravity pretty much and the fact that I'm still loosing weight... The problem is, is when she made sure the baby was okay they checked the heartbeat and it's only 144 BPM... I was just at my doctors on Friday and it was 150 BPM.. Is this normal? What can I do to make sure my lil one isn't hurt?