I will start by saying I am by no means an expert on sheep. I had my first 'solo' lambing this year and was as twitchy and nervous as anything. My answer MAY be way off the mark, but to me it seems logical.
I also noticed my ewes seemed to be straining slightly for a few days after they had lambed, a month ago. This was on a scale, 10% as powerful as the birthing straining. They also seemed to be seeping a little blood for a few days.
I related this to when I had my own children and what was happening to my body! We are all mammals and are therefore basically the same, so it seems rational that we will experience similar responses to things like birth, illness etc. I have human medical training...
After the birth the womb had to return to its normal size. So it keeps contracting down by the muscles squeezing and tightening. The feeling is a strong cramping somewhere half way between a period cramp and the early labour pains. Sometimes it was enough to stop me in my tracks! This could be what is happening to the ewes!!??
Also, ref the seeping blood... the placenta site is an interface of blood vessels which has been severed during birth. This site has to heal,... will heal, but takes a few days...or in humans weeks!
As with any mammal there is the chance that a bit of placenta has been retained. In some ways, i watched more diligently for the placenta than I did for the birthing. All the ewes delivered a large placenta, which probably weighed a couple of pounds, after the last lamb was born. They also strain for the delivery of the placenta.
If the ewes strain a lot for a long time, and you know there has definitely not been a placenta delivered get your vet, or shepherd guru!
If the ewes go really off colour over the ensuing days....get the vet to check for retained placenta parts.
I really hope this helps, and that you are enjoying your experience. I did! Its amazing and so rewarding!
BTW I took the MAXIMUM OBSERVATION, MINIMUM INTERVENTION saying to heart. And when I did have to go in I made sure I was well prepared, surgically clean, well lubricated, and that I took my time. Panic and dirt are your enemy!
Emma x