Ay, normal lang yan, mamsh! Yung "sagging" ng mattress ay nangyayari talaga over time dahil sa weight at pressure, lalo na sa side kung saan regular natutulog ang partner mo, nothing to worry about, it's just wear and tear. A few of us moms noticed the same thing sa mattresses namin after a few years, so it's really just how they age, especially if ginamit siya every night ng heavy person or couple. If gusto mo i-extend ang buhay ng mattress, pwede mong i-flip regularly or lagyan ng mattress protector, pero kung sobrang sagging na talaga, baka time na for replacement na lang!