Sick flock help! Coughing and runny poop

It started yesterday with one hen who was coughing and had lots of yellow thick poop stuck to her back end. Now quite a few of our 35 hens are coughing and have runny poop! they are normally free range during the day, but it's been cold and snowing lately so I've had to keep them in the coop. When they try to make their noises, it's like they are choking on their noises if that makes sense.

We just gave them electrolytes and we will out fresh bedding in tomorrow. We also have month old chicks in the same coop,'but where they can't contact the hens. Should we treat them as well? We just started up our flock a few months ago and are brand new to chickens. Any help is very appreciated!

Welcome to BYC!
It sounds like a respiratory infection. There are several that exhibits symptoms that you describe.
Can have one tested by a vet?

Hopefully someone with more experience/knowledge of illnesses will weigh in.

I know it is cold, but you may want to open some ventilation to help bring in fresh air if possible.
Did your chickens get sick after the chicks arrived - where did they come from, private individual, hatchery etc.?
If the chicks aren't showing any signs of illness, then you may only want to treat those who are ill. If you can separate the sick ones that will most likely be best. All of the respiratory illness are considered contagious and they will be carriers for life.

It could be Infectious Laryngotracheitis (ILT), Infectious Bronchitis (IB), Mycoplasma synoviae (MS, infectious synovitis) or one of the others listed in the link below (there is a comparison chart also).
http://edis.ifas.ufl.edu/ps044

It sounds like you will need to treat them with antibiotics which can be found at the local feed store, poultry site lists some of the medications which may be of help:
http://www.thepoultrysite.com/diseaseinfo/83/infectious-laryngotracheitis-ilt/
http://www.thepoultrysite.com/diseaseinfo/78/infectious-bronchitis-ib/
http://www.thepoultrysite.com/disea...tion-mg-chronic-respiratory-disease-chickens/