HCG not doubling

Kara
Hi everyone! This is my first pregnancy- after two years of TTC, I am very anxious and nervous. 
With my calculations I have just hit the 5week mark. This past Monday, my HCG levels were 2900. On Thursday, they retested and my levels were 4900. The doctor said since the numbers are double that I'll need bloodwork again Monday. 
What does it mean when your levels aren't doubling? They also set another ultrasound for Monday, which would make #2. Could they be trying to rule out an ectopic pregnancy?