Good evening - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that you've had a rough evening with Miss April!
Boston Terriers are what we call "brachycephalic" dogs - that's how we describe the short smooshed in noses. Brachycephalic dogs have variable degrees of airway obstruction, depending on how flat their faces are, how long their soft palates are, and whether or not they also have swelling in their larynx. Dogs that have significant upper airway obstruction can easily become heat stressed in situations that dogs with longer noses would take in stride. This is because their only means of blowing off heat is panting, and when they are breathing heavily, their airway obstruction prevents them from breathing effectively, and they can very easily become overheated and their core body temperature can rise.
It's important to check a rectal temperature on any dog that you think is having heat stress, and for every hour or two after they've recovered. If the rectal temperature is over 105, I typically begin cooling measures like applying alcohol to the paw pads and giving a luke warm bath. For the future, keep in mind that cold water baths are not recommended for heat stress. The cold water causes the capillaries of the skin to constrict, which causes blood to be held inside the body, thus causing the core temperature to actually go up. Luke warm is much safer. Also, watch out for overshooting the mark and making her temperature too low - sometimes these guys bottom out after their bath and we have to keep them wrapped in blankets until they dry. After their temp is normal, it's a matter of keeping it normal and keeping them from becoming stressed again.
In the early stages of a panting episode like that, sometimes a fan blowing directly into their face can help as well. Obviously in severe episodes, a trip to the vet is the safest way to go. In severe cases, brachycephalic airway syndrome can be life threatening if it's not treated with sedation and anti-inflammatories.
Please let me know what other questions I can answer for you:)