Please note that I do not have a medical degree … The blood from normal menstruation is very small comparatively and shouldn’t’ be impacted by fasting. But hormones are sometimes all over the place and that could be impacting you.
Have you been lightheaded during your cycle before?
I don’t believe it is dangerous in the sense of fasting, but if you’re getting dizzy and lightheaded, you could pass out and injure yourself, which is a risk. Perhaps look at breaking your fast a bit earlier when you’re on your cycle… menstruation is only a couple of rough days, so maybe the day before you’re scheduled to start, have something more nutritious, and the day you start, adjust your fasting times to a little earlier for those few days when you know you feel worse.