forgive my english not so good but i am assuming you said you someones punch is strong enough to hit your guard and you fell is it a knockdown?
I think every punch above the belt (even if its a shoulder) wich makes other fighter to fall down on the floor becouse of loosing balance/ getting hurt, is a knockdown. Its up to the 3rd man in the ring to determine was it a punch, push or a slip.
It's not cut and dry, It's really all up to the referee. If anything other than the soles of yor feet touch the canvass, the ref can rule it a knock down. Perfect example of this is taking a knee. There was a Prince Naz fight, where Naz was clowning, his glove grazed the canvass and the ref called it a knock down. He was well within his rights to do so. The ref also has the discretion to rule "non-knockdowns", such as slips, or if you are pushed. My advice, try and stay on your feet!
