Hcg level of 21 at 4 weeks

I just got a call from my doctor that my hcg level is only 21, I should be about 4weeks and 3days and he said its low so I have to go for another test tomorrow to see if it's increasing. I'm devastated, and incredibly worried. I know that the doubling matters more than the original level but I have such a bad feeling. Did anyone start with a low level and end up having an increase?