Hi VAsleeper,
Thanks for update #2 
Post #2 here has more information about the break in (for the mattress) and adjustment period (for the person on the mattress) of a new mattress. Both of these will often take longer than 2 weeks … particularly if your mattress has higher quality foam in it. Memory foam has very low resilience so it wouldn’t be “pushing back” on your back (just like a floor can feel firm under the areas of contact but doesn’t push back) but a new mattress will also distribute pressure across the body surface differently from what you are used to which is part of why a mattress needs to conform to your body shape. A mattress that is more supportive under the lumbar curve is usually a “good thing” … at least within reason … and the feeling you are describing is often part of the adjustment period for any new mattress. The last thing you would want is no support under the lumbar curve (usually caused by a mattress with comfort layers that are too firm or thin so you don’t sink in far enough to fill in the gaps in your sleeping profile and help maintain the natural spinal alignment).
Of course this could also be related to your medical issues which could lead you to be more sensitive to changes in your sleeping surface or experience pain and discomfort that are the result of your back issues themselves rather than the mattress.
Yes … if you have a box spring that flexes then this could make a significant difference as well. A memory foam mattress is designed to work best on a rigid non flexing foundation and if your boxspring has springs in it then it would be contributing to allowing your heavier areas (pelvis/hips) to sink in too far which could add pressure to the lumbar spine. You could test this by putting the mattress on a non flexing surface like the floor and sleeping on it for a few days to see what difference it makes.
The first (and best) suggestion though when you have any questions or concerns about a mattress is to talk with the manufacturer or retailer where you purchased it who will generally be able to provide you with some good guidance and insight based on their knowledge and experience with their mattresses.
Phoenix