The bleed you get when you are on the pill is not a period.
A period happens as a result of natural fluctuations in your body’s endogenous hormones over the course of your menstrual cycle.
When you take the pill, you are ingesting synthetic exogenous hormones. This down regulates your body’s natural endogenous production of oestrogen and progesterone.
When you have a break in your pill packet or you have placebo pills, you often get what is called a withdrawal bleed as a result of you not taking in those hormones for ‘X’ number of days - however many days your pill break is for.
This bleed is not because of natural fluctuations in your bodies endogenous hormones (the hormones your body produces) and is therefore not a natural period..it’s a result of you taking hormonal birth control.
For the above reason, if you are on the pill you cannot use the presence of a bleed as an indicator of health/that you are in a state of sufficient energy availability.
This is something to take into account especially if you are dieting to extreme levels of condition. You cannot make assumptions such as ‘I’m healthy because I have a bleed’ when that bleed is not a result of natural endogenous hormonal fluctuations throughout the course of a menstrual cycle like it would be if you were not on hormonal birth control.