Hi L.! My 2 yr old son has this as well. I noticed it when he was about 5 months old and took him to our pediatrician immediately. At the time it was very noticeable and she sent us to an orthopedic specialist. We took him in and sure enough he has infantile scoliosis--x-rays confirmed. However, the specialist said it was very mild and was surprised that we had even noticed it. But to us, it looked severe. One of his shoulders was higher than the other. It's interesting what you said about the swing b/c my son also slept in his swing at night--not rocking though but still in the swing. He slept there until he was about 5 months old. I, too, asked the doctors about this and they all said that it wasn't b/c of the swing but I always wondered.
Anyway, since his curve was not severe--it has to be a certain number--he decided to keep an eye on it and felt that it was straighten out on it's own. We were relieved to not have to put him in a brace. I don't think they do that anyway until they are at least 1yrs old. So we have been going back every 6 months to do more x-rays and sure enough it is correcting itself. We had gone back once and there wasn't any change but the rest of the time, it has been straightening out. We just went last week and since he's looking so much better, they don't want to see us for 9 months and that will probably be the last time until puberty. Apparently, you have to watch it during puberty.
I also want to suggest, as the previous posters, about going to the chiropractor. We took him in and strongly believe that it has helped straighten him out as well. He's only been a handful of times but still made a difference. I plan to take him back soon, it's been awhile.
Oh--don't be surprised if your daughter walks really late. My son didn't end up walking until he was about 20 months old!! The orthopedic said this was normal but he would be walking by 2 yrs old. Which he did but it was really late and freaked me out a bit.
Anyway, I hope this helped!! Hang in there and feel free to contact me for any support/advice! I'm not sure if my email is shown on here??