Confused. Is it still safe to say you aren't pregnant if... read below!

Okay recently I heard that when on the pill, if you get pregnant you won't know for several months because you will still have the withdrawal bleeding.

So, if my withdrawal bleeding this month was super heavy. Filled pads over several days. Bright red. Lots of clots.

Would this still be considered normal for someone the pill while pregnant????

I don't see how its different and why that bleeding would be considered okay in early pregnancy if it's still emptying your uterus?