Hi zexpress,
Unfortunately, I can’t diagnose what specifically may be causing your issue, but I can provide some general guidelines that I hope will be helpful.
Low back discomfort can be caused by a mattresses that are either “too firm” or “too soft” on the surface (common causes of pain when sleeping upon a mattress are listed in post #2 here). If the mattress is too soft on top or not supportive enough, it could allow you to sink in too deeply and allow for excessive sideways bending of our lumbar/thoracic region. If the mattress is too firm, it may not allow your hips to sink in quite enough and this could leave less support for your lumbar region, which can then allow an excessive lateral bend in this area.
As you stated you sleep upon your stomach at times, this can also cause some lower back issues, as the surface comfort generally preferred for someone who sleeps prone should normally be a bit firmer, as to minimize the accentuation of the forward (lordotic) curve of this area. And of course this all depends upon your somatotype and levels of strength and flexibility.
As this issue just crept up, you may also wish to reflect and see if there are any differences that have happened to your recently during your activity or workday that could be contributing to this issue. While you are still in the process of adjusting to your new mattress, and the mattress itself losing some of its false firmness, at two months the majority of that should have taken place.
Quite often a short-term morning soreness is attributed to a firmer surface comfort, and as this mattress was perceived as quite hard to you when new, it may be that you desire just a bit of extra plushness on the surface of the product. If things don’t improve, my best advice would be a phone call directly to Novosbed and explain to them your symptoms and see what advice they provide. Their Comfort+ layer is not memory foam, and it has helped people sleep better in situations similar to yours. But I’d leave that final advice up to Novosbed.
Phoenix