Negative Test

I was previously on Depo shots prior to ttc. My last 2 periods though have arrived on the 4 week but this time I'm on 5w3d and still no sign of af. I took pregnancy test Monday and today but both were negative. Is it possible that my hcg is still low and home test just can't pick it up yet? I'm going to doc today for blood test. 
The test I took was the Clear Blue digital with Conception Indicator... I do not like it, I'll be sticking with First Response.