If her sides are sunken then she hasn't enough food in there. Ruminants need to eat or their digestion goes wrong and they become ill very quickly.
Best option is vet, pronto.
If that isn't an option, as a stopgap I might give her a drench of yoghurt, honey, ginger, keep her in with some really lovely hay, and try to tempt her with oatcakes or whatever your sheep have as a treat.
If she's mucky at the back, then worming may be indicated.
If you get her in, put her on clean straw so that you can see what, if anything, is coming out the backend.
Lethargy and not eating could be fluke, could be lacking minerals. Maybe add some Lucozade to that drench, or give her a Twin Lamb drench.
Then vet on Monday.
Good luck