negative tests...i dont wanna be alone

has anyone ever peed on a test and have it come out negative even though it should have been positive. I was 6wks with my daughter and didn't know it till I had blood work. 15weeks in with my daughter I had gone to the ER for extreme back pain the docs took a urine sample and it was negative but my blood was positive. now here it is 4hrs later our daughter is here and about to be 5. Now we're 2yrs in TTC and nothing still I still have to get blood work to make sure. my question is does anyone else have to get blood work because their HCG doesnt read urine samples please tell me I'm not alone!