Saggy piglet?

One of my piglets (around 10 days old) will sometimes sag in the back. She doesn't seem distressed by it, doesn't seem ill, doesn't seem to be in any pain. Her back legs will sag then she'll get up and walk or run with the others. It mostly happens when she stops walking for a bit. This is the second day I've noticed it. I looked up white muscle, navel ill, dippy pig and none of those seem to fit. Could she be injured? The others will sometimes bite at the piglets when they're in their way. Or is there something I need to do, i.e. minerals? She's white with black on her face, potbelly. And the runt of the litter. One site said it could be from the sun, almost a fainting kind of thing, that happens to white pigs, but she doesn't go all the way down, just in the back.

I did more reading and it's sunburn. Odd that it makes them get like that but several sources said it does. The cure is to give them more shade and maybe spray-on sunscreen. The area they're in has morning shade/afternoon sun and sure enough it only happens in the afternoon when they've been in the sun for a few hours.