Confused and scared

Yesterday I went to get an u/s and they said that the heartbeat was 164 but the sac was a little on the small side. However, my OB called me today saying that she is worried because they told her that the heartbeat was 117, which is low, but that the sac looks good it's small but nothing to worry about. I'm just so confused so is this bad? Having slower than 7weeks? But when I first found out I was pregnant it took a little while for my HCG levels to rise, so is this what is happening with my pregnancy that is taking longer than usual. Please help very scared