Is it normal to feel slightly warmer than normal if it's been 23 days since your last period and your next one is due to start within the next week?

Yes. Body temperature is higher in the second half of the menstrual cycle, following ovulation, due to secretion of progesterone. Visit this site for info: https://www.plannedparenthood.org/learn/birth-control/fertility-awareness/whats-temperature-method-fams#:~:text=Your%20body%20temperature%20naturally%20changes,their%20typical%20temperature%20before%20ovulation.