HCG levels triple in 48 hours?

Well i finally got my BFP....yay!!! I am very nervous since last ime i miscarried after one week. SO far, everthing is on track and i have made it to a gestational age of 4 weeks. My doctor is monitoring my HCG levels closley and testing every two days. My first test was 35, two days later i tested and it was already up to 96! I thought levels were supposed to double every two days, not almost tripple...anyone have any experience with this? Is this normal?