How many times can you breed a female dwarf lop eared rabbit healthily in her lifetime?

I have had dwarf lop eared rabbits in the past and bred them but the two females I had died when they reached five. I have read about uterine cancer but they showed no symptoms. One was fat and had one dead baby and I thought that was all she was going to have but one got stuck and she (the mum) died. The other one wasn't fat and died a day after giving birth to 4 babies, one died from diarrhoea and I successfully hand raised the other three on a milk formula from my local vet because they wanted to suck out of a bottle. But I did try them on 50 to 50 of full fat milk and strawberry yogurt, they are all females. I have got another female and a male and wish to breed them when they are old enough but I wanted to know how many times you can breed them females in a healthy way and how old should they be when I stop breeding them. Do female rabbits die at the age of five if bred or was their something wrong with them?