HCG levels 15 at 4 weeks pregnant?

Just need some advice and information on this. I am currently 4 weeks 1 day pregnant based on my LMP. I got a positive HPT at 2 weeks 6 days and had a blood test the following day that came back very low. I did another HPT at 4 weeks and that too came back positive. I had a blood test the same day and it came back as a HCG reading of 15. Doc wants me to have ANOTHER blood test in a day or two as she said this is uncommon and can't give me a definitive answer as to anything :( I went to another doctor and he has booked me for an ultrasound at 4 weeks 5 days. This is my second pregnancy. TIA :)