If you go to Houston in May, you probably won't notice much difference in the heat or humidity from what you're used to in Chicago. It hasn't even STARTED to get hot in Houston in May!
If you go to Houston during the hot months of July, August and September, the outside temperature can be over 100 degrees every day, and the humidity makes it even worse. If you're unused to it, you'll probably suffer during those months.
If you can stay inside during the hottest part of the day during the hot summer months, you'll be okay because the hotel rooms and all public buildings are air-conditioned, 24 hours a day. Autos are air-conditioned; cabs are air-conditioned; city buses are air-conditioned; restaurants are air-conditioned.
EVERYTHING in Houston is air-conditioned all year round, and especially during the hot summer months.