Hcg doubling but progesterone fluctuating?

Hello ladies, my hcg at 5 week 4 days was 21,000 which is doubling and I even saw the heartbeat of baby. But my progesterone levels have decreased from 24 (4weeks) to 28 (5 weeks) to 18 (6 weeks). I am not sure what is happening. Please some help would be just what I need.