Hi aron82e,
I’ll add my own thoughts which mirror those of TD-Max as well.
First of all I would also agree that it’s not “in your head”.
When you buy a new mattress … the mattress itself will usually be somewhat different from the one that you tested in the store because all mattress materials … particularly foam and fiber … will go through some initial changes (softening in the case of foam, compressing in the case of fiber, and stretching in the case of fabrics).
In addition to this … there is also an adjustment period where the body adjusts to the differences between the new mattress and the old one you were used to. The bigger the difference between the new mattress and the one you purchased … the longer this can take (in most cases it is a matter of a few days to a few weeks or so). Part of the reason for this is because of the difference between “natural” alignment and “learned” alignment and the body memory that develops as muscles, ligaments and joints that may have become “locked in” to certain positions loosen and stretch and gradually adapt to the new surface. There is more about this in the first part of post #7 here. This can and often does happen because of the breaking in process of the mattress itself but even when the new sleeping surface is better in terms of pressure relief and alignment than the old one, any change both positive or negative can have some “symptoms” connected with the change itself.
If this continues over the longer term or gets worse (whether from the breaking in of the mattress or because your body doesn’t seem to be adjusting to the new mattress) … then it can also be an indication that you may have made a choice that doesn’t match your needs and preferences … but if it does, then it’s just a matter of time as muscles, ligaments, and joints “re-learn” a better and more “neutral” or natural position.
Phoenix