Sounds like you have a mystery...
...I don't like mystery's when it comes to losing calves.
...we had one a few years back that we lost, cow got it's leg stuck in a gopher hole (or something) and was limping so we brought her and her 3 month heifer calf up into a smaller pen (40' x 60') for a week so she wouldn't have to travel as far (no foot rot, nothing broken, no evidence of injury on the foot or ankle of the cow)
2 days later the calf was dead -- we always figured the cow lost it's balance and fell on it but we'll never know for sure