Hi radorix,
There is more about the most common symptoms that people experience on a mattress and some of the most likely reasons for them in post #2 here but the most common reason for lower back pain is a mattress that is too soft and allows your hips/pelvis to sink down too far and puts your lumbar spine out of alignment over the course of the night. This is a common issue with softer pillowtops.
Another common issue with pillowtops is that they often use lower quality materials in the pillow top which can soften or sag prematurely so if you don’t have the back pain when the mattress is new it can appear relatively quickly once the materials start to soften. Most of the major brand mattresses use lower quality materials in the comfort layers (including the pillowtop) so I would always make sure that you know the type and quality of the materials in a pillowtop before buying a mattress to make sure that it doesn’t have lower quality materials which would be a “weak link” in the mattress in terms of durability (see this article).
There are also some suggestions in post #4 here that may be helpful but hopefully you will be able to exchange the mattress for something that is a better “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) rather than needing to “fix” a mattress that may not be suitable for you.
Phoenix