The basal body temperature shift after ovulation is due to

Your body temperature dips a bit just before your ovary releases an egg. Then, 24 hours after the egg's release, your temperature rises and stays up for several days. Before ovulation, a woman's BBT averages between36.1degC (97degF) and36.4degC (97.5degF). After ovulation, it rises to36.4degC (97.6degF) to37degC (98.6degF). Ref: guyton and hall textbook of medical physiology 12 edition page number:636,637,638