2 questions about my pigs

hi there  I am just starting out with my first feeder pigs. I feed purina hog grower from my feed mill  does anyone use this product  I cant find information anywhere on the amount of feed to feed my pigs. I asked the feed mill and they said free choice but this seems impossible to me and I have heard that feeding too much will create a very fatty carcass. thoughts

I have also observed my pigs doing something  find abnormal, but I haven't found any other information about it online. Background: they have been with me for 2 weeks, they lived indoors and ate a different feed prior to coming to me, now they are on pasture and unfortunately have gotten a sunburn, now in have seen them reverse arching their backs and sometimes even dropping their bellies to the ground and walking along as if they cant use their back legs. It is only very intermittently, I am thinking maybe they are scratching but it doesn't really explain why they are making their backs concave at times as they walk along, almost a humping motion. anyone experience this, is it normal, should I worry  thanks