Thanks robert. I wanted someone with loads of experience to tell me that.
She's eaten and behaved normally today, the piglets seem fine (though they haven't come out of the ark very much, we've had just awful weather today), and she definitely has milk - I squirted some while she had her head in her feed.
I think what probably happened was she was hungry (I was later with her midday meal than usual), the weather was wet and horrid so rootling was less appealing and the straw in the ark was damp and muddy from the tramping in and out in wet weather - and she ate too much of that damp muddy straw and made herself sick. It doesn't explain the piglet throwing up - but maybe that happens fairly often, we just don't usually get to see it as we don't usually have the luxury of having the time and/or reason to sit with them for long periods !
My book (the useless Andy Case one - I must get me a better one..) implied that vomiting was unusual and worrisome, and that a low temperature was seriously bad news, and I knew from stories played out on here that you don't want to mess about with lactating sows, but get her sorted quick. So I called the vet. He has a proper mercury thermometer so got a better temperature reading - still a bit low but not so bad. He also thought vomiting was unusual, but was honest enough to say he doesn't get to see a huge number of pigs. So I remained really quite worried until she seemed normal today. It's a relief to hear from someone with robert's experience that vomiting is not unusual in either sow or piglet.