My python's throat keeps swelling up and there are clear bubbles coming from his nose and mouth -please help!!

I just got him one week ago. He is a 4 year old ball python. He is still active but he just sounds and looks stuffy and congested (he wasn't like that at the petstore and I the guy I bought him from I know and trust). I have not fed him yet, but I plan on feeding him this weekend. I am using repta bark and a heating pad under the aquarium, as well as a 60 watt lamp beside the aqauarium for extra heat/light during the day. I have also been boiling water to try and raise the humidity in the house because it is dry and cold in South Carolina right now (I also mist him once or twice a day with warm water). I have heard that there is a medicine of some kind for reptile respiratory infections at PetSmart or something... is that true? I just need some help and advice because I am not an experience reptile owner yet and I am worried about my snake :(