There's probably going to be a hundred different reasons/trains of thought and sadly, I don't know if you'll be able to find out exactly what's caused it.
I wouldn't have thought the litter size would drop significantly after three litters unless there's been a big gap since last litter, an illness/infection/stress or she's gotten really fat (can affect the amount of eggs released)
If she was moved within 4 weeks of service it can affect embryo implantation.
If the boar wasn't stimulating her pre-service, nudging her belly to encourage egg release that could be a factor, as could diseases. Was the boar serving lots of gilts/sows ? Was it a different boar ?
Do you vaccinate for erysepilas ?
Was the whole service/pregnancy the same as the last one or has anything changed ?
It's entirely possible (and quite likely given the piglet eating) that she's eaten the placenta's. But I don't know about the placenta's decomposing & coming out as liquid
Definately get anti-biotics into her, this will ensure there's no chance of an infection setting in internally.
Sorry for your losses
Karen x