I got false negs with both of my girls and one invalid. It could be not enough urine as you said or some got into the control section. Try another brand , like the digital ones or first response. First thing in the morning is best especially if you have low hct levels. Also, make sure to hold it in the stream from the begining, the correct length of time, and not to splash into control. Then lay flat. Or pee into a small cup and dip tester into it. Then lay flat.
Still not sure see the doc for a blood test. Either way GOOD LUCK!