Is this a mattress or a sink hole?

Serta iComfort Prodigy mattress set was purchased for my husband (175 lbs) and myslef (120 lbs). The mattress seems very well made and was soooo supportive and comfortable for the first month, but then... in the 5th week we started sinking (like a hammock as another reviewer put it). When I lay on my side it feels like I'm sleeping on a hill, lol. Side sleeping also causes strain on my knees because my hip sinks way down and my knees sit on the top of the mattress. In both scenarios (on back and on side), I wake up with terrible lower back & hip pain because the mattress has lost the wonderful supportive qualities it once had. I slept on the mattress for approximately 6-7 weeks of the 9-1/2 months we've owned it. Every couple of weeks I'd unsuccessfully try sleeping on the mattress again for a night. I always ended up waking after a few hours with severe back/hip pain and going back to our guest bed to sleep. It seems like I sink more and more as the night goes on. Maybe it's caused by the warmth of my body heat? Our room temperature is usually about 68 degrees (so it isn't overly warm). Although, I think I remember the salesperson telling us that room temperature wouldn't matter with this mattress. To the reviewer who has a really firm bed, I'll trade ya, lol. Finally fed up with not being able to sleep on my brand new mattress, my husband called the store we purchased the mattress from to tell them of our situation. Thankfully they are sending someone out tomorrow to check it out. I can't wait! I'll post an update after they leave to let you all know if this is how the bed is supposed to break in or if we qualify for a replacement. 9/19/12 update: The gentleman came and took measurements of our mattress this morning. He said we will here back from Serta within 7-10 days. 9/25/12 update: A few days ago I submitted this same review on Serta's website. On 9/24/12 I received a confirmation e-mail from Serta that "My review has been approved." I went to their website and sure enough it was posted. I went back today (9/25/12) to see if anyone commented and it's gone! The only reviews on their website are positive reviews. Shame on you Serta. As a consumer, I want to read the good & the bad to make an educated buying decision. This makes me very leery about reading reviews ONLY from their website. Thank you goodbeds.com for providing a forum where both good & bad reviews are posted!