What day do you count as your first day of your period?

So i have always been confused as to which day is actually the first day of being on my period. I always thought that if you bleed even a little, and then it comes down regular even if its the next day, that day is your first day of your period. But then again some girls call that first day spotting? For example, last night i had sex. After i peed i seen a little blood almost nothing. So i thought maybe that was a little rough sex, idk. Now today, hours after having sex, i go to the bathroom and boom, AF comes down. So yesterday when i bled that little bit almost nothing... thats spotting im assuming. So today is my first day? Right? Sorry if i sound dumb but if my cycles are never on point, im trying to understand my cycles and my ovulation dates but its pretty hard when im irregular. Please and thank you!