What is a normal cycle : The days in between periods

Good morning ladies 💗

I am noticing that for the last 3 cycles they have all been 20-21 days apart and only 3-4 days max in length. The first day is light bleeding, second day is usually a little heavier then the third day it tampers off and or the 4th. I’m unsure if this is normal, as it seems irregular compared to what my periods have normally been. I just wanted to know if anyone has experienced a shorter cycles / shorter lengths in between and what can cause this.