One of 3 new chicks is lathargic and sneezing!

We just got 3 four week old chicks from our local feed store. Weve had them for about a week. I have them in a large bin with hay in the bottom, fresh food and water on one side and a heat lamp on the other with screen over the top. We are in MN and it's still pretty cold here at night (30s). They are in our garage, protected from drafts but the one that's a little smaller than the other 2 has become lethargic and is sneezing a little and shaking her head from time to time. I still see her eat and drink but she definitely looks like she doesn't feel well and falls asleep standing up next to the feeder. Ive never vaccinated chicks before and we don't give medicated feed. We are separating her from the other two once we get another set up going tomorrow but wondering beyond separation and obviously keeping her environment warm and stress free what should i do for her?

 

Thank you!