Hcg levels rising but not quite doubling

Hi ladies! I had my first doctor's appointment on Monday and am roughly 5 weeks. She saw me so early because I wasn't exactly sure of my LMP, as I had a weird period in December we weren't sure if it was truly a period or not. 

Anyway, she did an ultrasound and determined I was around 5 weeks and then she drew blood and told me to make an appointment for Wednesday for follow-up blood work to make sure my HCG levels were rising appropriately. She called me this afternoon to discuss the results:

On Monday 2/13 I had blood drawn at around 2:30pm and my hcg level came back around 3500. 

On Wednesday 2/15 blood drawn around 9:45am and my level was 5952. 

Initially I was supposed to go back to see her on March1, but after I spoke to her today she wants me to come in next week too since my hcg level didn't exactly double. She said they like to see levels increase by 60% and said that mine went up by 68% and that I shouldn't be worried. She said that everything looks like it is progressing and that my progesterone looks great, but naturally I am a little nervous as this is my first pregnancy. 

I guess my question is, is it normal for levels to not always double in just under 48 hours? Thanks for the help and best wishes to all the other October moms!!