my hen is acting lethargic? PLEASE HELP

I've had some weird occurrences lately with my black Australorp hen. She hasn't layed in almost a month, she doesn't seem to be showing any signs of being egg bound (I felt around her vent and all seemed normal). The past three weeks she would act on and off "lazier" than usual then bounce right back to herself within a day or two. This morning I noticed she appeared quite lethargic, standing to the side, not engaging with any of the other chickens and not interested in the treats I brought out to the coop. I suspected mites but my chickens are in sand bedding and dust bathe frequently, and I didn't see anything that looked like an external parasite on her. I did notice, however, that she seems to have lost some weight and the bones slightly below her vent and to the side are more prominent. Also, we have been having some very hot and humid weather lately, not sure if that is causing anything but I am sure it isn't helping. Also her poop looks a bit odd.
I decided to get a dewormer and probiotics because I suspect she may have worms.

I do not know exactly what is wrong with her so any information or suggestions on what I should do are greatly appreciated. Thanks