So Frustrated and Discouraged :(

I have been constantly getting faint lines on the dip strip tests (yes I know I need to go to the Dr. and get blood work done). They are medical grade as I am a nurse and my boss told me to take some. I got a vvvfl yesterday. I took a Wal Mart test today (FMU) and got what I assume is a bfn. I had to do some major tweaking to see anything. So my question is....is there that big of a difference between test brands? Or did I have a chemical? I know they say FMU is best, but I seem to have stronger lines with afternoon or evening urine.