I had some recent issues... sleep in a king size bed, and noticed I was waking up with a bad backache after 8 yrs of sleeping the best sleep ever on my Simmons mattress (now out of business). It felt like there was a divet in the mattress.  That was the first time I'd ever spent that much on a bed.  In going thru some old paper work, found the receipt for it and realized it had a 10 yr warranty and I was just going into year 8..  so I called the company from where I purchased it,  and they had a service person come out, inspect my bed, take pics, measured the 'sag' and spent about 15 mins checking everything out.  It ended up that the matress had slipped off the center divider of the king bed frame, enough that it wasn't supporting my back. He pushed it back on the frame, suggested I take a couple old paperbacks and tuck them inside the frame on the siderail to secure the box spring .. sure enough.. it worked like a charm . No more backaches! And it didn't cost me a dime for them to come check it out.

  Check to make sure your bed is securely in its frame. I felt kinda dumb afterward because I wouldn't have thought to check the frame. Good Luck!