EXTREMELY Low Rising HCG Levels

Courtney
I found out I was pregnant on 3/6 after ttc for 4 years. My dr ordered lab work to check my hcg levels. Can anyone provide me any insight as to what is going on and what I should expect. 
HCG Levels:
3/7: 61
3/9: 87
3/13: 131
3/15: 196
I understand that the levels should double every 2-3 days and mine aren't. I'm 5 weeks 5 days. I would appreciate any help or advice.