Ugh so you can’t be sure she didn’t retain some placenta. I’d keep a close eye on her and call the vet if she seems to be worsening at all. In the meantime would you like me to reach out to some farming friends for more specific advice? Unfortunately I know people who’ve raised goats and cows and not sheep. I’d try a sheep specific message board. Have any neighbors raising sheep you could ask? 

 Do you have her separate from the flock and under watch at least? 

 Yes she’s under close watch. Been out with her every few hours. 

 Do you think it’s lambing sickness or milk fever? 

 She also could have eaten it.
Very very common.
If our gray DIDN’T eat the placenta I would have been worried. 

 I did actually wonder if they would. But then I’d also be concerned as to what caused her quick decline 

 I would be confident that the ewe eating her own placenta is not what caused her quick decline.

But the other responses you received are possibilities.

Depending on how badly you want to know, many vets do autopsies, but it usually costs quite a bit.