Weird?! Wrong blood test???

Ok so I went to urgent care because my doc refused to do a blood test (they refuse to do them in general bc it’s a military health clinic and “it’s expensive”). The results came back negative. This was at 15dpo( pretty sure). I am not 18dpo and four days late. I had another test laying around so I took it with zero hope bc a blood test is a pretty sure thing. Well it came up a faint positive?. Why the heck would I have a negative blood test at 15dpo and positive pee at 18dpo?