I have low hcg levels and am 6weeks and 2 days pregnant baby is measuring 5weeks and 6days and a heart beat has just started but the sac is measuring smaller then the baby what does this mean?

I have low hcg levels and am 6weeks and 2 days pregant baby is measuring 5weeks and 6days and a heart beat has just started but sac is measuring smaller than the baby.also brownspotting has just started what does mean ?????