Well, I have been having similar problems with my Moshu. Two weeks back she used to be active enough to make me cry, but now she is lying on the floor 80 % of the day. Besides lethargy, she is also showing a slight decrease in her appetite. Decrease in activity and appetite isnt a good combination. I am taking mine to a vet on Saturday (I am suspecting there is something wrong with the hay I gave her, though I do tend to be a worrywart, especially when it comes to her)
If vet bills are an issue, I can totally empathize with that since I face a similar situation. You may want to wait a few days. Sometimes Moshu’s poops are softer than they should be and she gets lethargic, but with the right food she gets back on track in a matter of 2-3 days.
Keep a check on her poops, her food and water intake, body language (read some articles on rabbit body language). But if her body language are showing signs of pain, try to take her to a vet. Since you mentioned its hot in Chicago these days, you can wet the back of her ears with water as that will help them reduce their body temperature. Try to feel her stomach. If it feels tight and bloated, you may want to read about GI statis.