It probably is pretty common for temps to be that much lower for that part of the month (although just because it’s common, I’m not sure if it means that it is optimal). 97.6 is probably still kind of low, but I think you have to ask yourself how you are feeling. Are you feeling symptoms of low metabolism- which could be so many things, really, but are you tired, do you have digestion problems, sleep problems, cold hands/feet, etc??
If you’re feeling great, I wouldn’t stress about it. If you’re curious and able to tweak some things to get those temps up a bit at that time of the month, you can see if you feel better or if it makes a difference.