I Don't Understand what Happened to my Cat's Kittens... ?

My cat had kittens on the 25 of March. She had three, rather adorable, little kittens that were quite chubby. When the first one came out, it squirmed and mewed and immediately started suckling. But the second two... They...were born chubby, but also... born dead. She had a few litters before, and every time she did she had four loud and fat healthy kittens, but the recent litter had only three and two were... you know. A long time ago, she got hit [carelessly] by a car and hurt her hind leg. But that was a long time ago [over two years], and she runs and jumps even better than she did before the accident, and she doesn't mind it when anyone touches her there. My grandma says that it's because of that. And it is her first litter since the accident. But I'm still confused... I fed everytime she wanted me to, and her water bowl was always full. I just don't understand...