Started to sink after a couple weeks

I was stoked on this mattress when I first bought it. I paid twice as much for it as I did on my previous mattress, which was also a memory foam. It started out firm and comfortable but after just a couple of weeks it started to lose that firmness. I've had it about 3 months now and it sinks when I lie on it and I wake up with a sore back. The warranty says that they will replace it if it loses more than 1" of it's height. Well, it's still 12" when I'm not lying on it; it's only when I'm lying on it that is sinks down. I'm pretty disappointed and thinking that maybe I should have paid twice as much for the Tempurpedic.