Hi chip,
A single extra firm support core would provide great primary support but it would probably be too firm for most side sleepers without a softer comfort layer in the range of 2" - 3".
It could be almost anything but most likely is that you are not used to sleeping in this position and your body may need time to get used to a different sleeping posture. Post #2 here may be helpful as well.
Normally it’s a good idea to wait a few weeks before assessing things because of both the break in period for the mattress and the adjustment period that is involved with a new mattress but if you spend a fair bit of time sleeping on your side then it’s quite likely that you may need some extra softness for pressure relief. There are some topper guidelines here (and in the post it links to) that may be helpful if you decide to add a topper to improve pressure relief…
Phoenix