Practically every one of us keeps feed continually with chicks. Don’t for a minute believe that about them overeating and getting an upset stomach.
It’s hard to tell what is going on at a distance over the internet. There are just too many things I can’t possibly know or you can tell me.
My first thought is Coccidiosis. Are yours being kept I a wet environment? That’s usually something that can lead to a Coccidiosis outbreak. Some strains of Coccidiosis are stronger than others so you don’t have to have something wet for them to come down with it, but wet makes it more likely. Look up the signs for Coccidiosis. Some people believe they have to be pooping blood for them to be suffering from Coccidiosis but that is not true. Them being puffed up and lethargic can be a sign. But do some research for a better description.
My next thought, especially with them panting, is how warm are they? How are you providing heat? I’m guessing that 30 C (86 F) is your daily high? What is your daily low if you are not providing heat? At 2 weeks they can handle a pretty good range of temperature but it might be cold related. I don’t think it is cold but I’m not there.
Opening their mouth is a way for them to try to cool off. Heat can kill. Are they trying to get as far as they can away from your heat source? Do they have a place they can go away from the heat source so they can cool down if they need to? I strongly advocate having a brooder big enough that you can heat one area but let the rest cool down a bunch so they can find their comfort zone.
Those are my quick initial thoughts. It could easily be something else. Good luck. These things can be frustrating.
