I need some understanding..

So you would think if you got a couple positive pregnancy test all of them would come up positive. Well that's not happening to me.. Only certain pregnancy tests are popping up positive. The rest are negative. The only brand I that has showed up positive are the walmart brand first response sticks.. So I'm not sure if I truly am pregnant or not.. I'm getting upset. I'm going to try to make an appointment to get a blood test done. Has this ever happened to anyone else & ended up being pregnant..???