The humidity/heat make breathing difficult, but also if you are on city water, alot of chemicals are added to kill bacteria and diseases in the water-these chemicals tend to cling to moisture like vapour particles which you then breathe in. Maybe it was all a bit much for you- or you are developing a sensitivity-we are bombarded with toxins in the air 24/7. Mention it next time you are at the doctors, go to the hospital if it happens and doesn't subside.