Hi Denver Shoulder Pain,
All mattresses will go through an initial break in period where a cover stretches and loses any initial stiffness it may have, fibers will compress and become a little firmer as they pack down, and foam materials will lose any of their initial "false firmness … but latex is the most durable of all the foam materials and will maintain its firmness longer and soften more slowly than any other foam material over time so it’s much more likely that your mattress was “on the edge” of being too soft for you and that even a small amount of foam softening was enough to put you “over the edge” (see post #2 here). In some cases it can also be a matter of time because if you are sleeping out of alignment then like any repetitive stress that is causing pain or discomfort … time itself can make a symptom worse or more evident than it was initially.
While most people would be fine inside a range of firmness options … you may be “jumping over” the combination that works best for you. If your mattress is too soft (either in the thickness/softness of the comfort layer or in the softness of the support corre) then some of the suggestions in post #4 here or in post #11 here about zoning may be helpful to “hold up” the heavier pelvis/hips to keep you in better alignment. If you have a 3" extra soft comfort layer then this may be too thick or soft in combination with a softer core and then if this is on the bottom the core itself may be firmer than you are comfortable with sleeping on directly and the extra soft 3" layer on the bottom in combination with the softer core may still not be firm enough to provide good alignment/support. If the only issue is that your mattress is too firm and not providing the pressure relief that you need then some of the suggestions in post #2 here (including the suggestions about different pillows) may be helpful as well.
I don’t know your weight or body type but I would have been tempted to keep the firmer core and then use fine tuning adjustments on top of it to help with any pressure issues because it’s generally easier to fine tune for comfort/pressure relief issues than it is to fine tune for support/alignment issues if the underlying issue is that your base layer is too soft for you.
If the topper is returnable then there would certainly be no harm in trying it but a topper is generally a pressure relief component and will have less effect on support/alignment although personal experience always “trumps” theory at a distance so your own experience will always be the best way to know how well any combination will work for you.
Phoenix