Urine vs. blood test

Earlier today I posted about my experience at the clinic. Long story short, home pregnancy tests were not showing me anything definitive and 5 days late, and the pregnancy test done at the clinic (urine) came back negative. (Cue sad feelings) But then my PCP ordered a blood test whose results weren't due back til Monday. Luckily, I was given a call some hours later today with the results, but they are confusing. The blood test did find HCG levels, but just barely. My count is 17.4. Ultra low, I know. The nurse said this is likely why the urine test came back negative. I'm supposed to go back Monday for another test to see if my count has gone up. I'm praying, praying that it does and that this pregnancy (if it can be even called that at this point) is a healthy one that goes full term. I'm hoping my count is so low due to it being so early...