I took mine in on Saturday for the same reason. I don't usually weight them, but had started when I got the babies a couple of months ago, just for interest, and had been including Hazel along with them. I'm pretty sure the last time I'd weighed her (for mite drops, about a year ago) she was about 1400g. At the beginning of Sept she was 1330g, but on the 23 Oct weighed in at 1131g. Despite having been concerned she had been on the too-plump side I was concerned about this drop of at least 15%.
So, in we went and her teeth were fine (he was surprised she had some food in her mouth, I wasn't, you never know when you might need a stash!) and she seems in fine condition. He did, however, find a lump in her belly which is concerning, but we have no way of knowing what it is unless she goes under, which, of course, is no laughing matter for pigs. The advice was to keep an eye on her and take her back if she loses any more.
Of course, when I double-checked this morning it turns out she has put weight on in the last two weeks (13%) so whilst she is still under what she was two months ago it's less than a 5% reduction. So fingers crossed.
(She would say that of COURSE she's lost weight, as her exercise regime has changed dramatically. It's VERY TIRING watching the babies run around all day, dontcha know!)