Negative test after positive blood work

Brittany

So I'm a little concerned, I've had two positive pregnancy tests, then went and had blood work done, confirming I was approx (2weeks) along, and just for fun I took another test I bought for a dollar and it said negative. I took this test in the evening after a lot (I mean a lot) of water, I'm going to take another one in the morning but Has anyone had this happen?