it depends on when you OVULATE (release of mature eggs).
Some people can ovulate during menstruation. Generally you will ovulate around 2 weeks before your menses come.
When you do BBT charting, you will see a sudden sharp sustained increase in body temp when ovulate.
Day 1 - 36.34
Day 2 - 36.42
Day 3 - 36.23
Day 4 - 36.38
Day 5 - 36.24
Day 6 - 37.01 (This is a sudden increase in temperature)
Day 7 - 36.98
Day 8 - 37.03
that means ovulate on Day 6, which might still be having menses for some people.
Provided you are NOT having a fever or feeling unwell.