Hi Joanne,
I think that you are going to Walt Disney World at a great time of year. In May, the weather will not be as hot as it can be in the summer, and since you are not going during any major school holidays, the theme parks should be not too crowded. The average high temperature in Orlando in May is about 88 degrees Fahrenheit, so it will be warm without being stifling. Great weather for taking a swim! You should find crowds to be about average. They might pick up a bit around Memorial Day, which is May 26 this year, so do be aware of that. When I am looking for predictions of crowds at Walt Disney World, I like to visit the unofficial website touringplans.com. For a small fee, you can see their predictions for crowd levels at Walt Disney World for a year.
There are a couple of special events going on during your stay. The
Epcot International Flower and Garden Festival will run from March 5 - May 18 of this year. If you can, you might want to check out some of the offerings there, such as the amazing topiary displays and outdoor kitchens, on your first couple of days of vacation. Also, on weekends from May 16 - June 8, Disney's Hollywood Studios will host
Star Wars Weekends. If you are a fan of Star Wars, this is a great event including appearances and autograph signings by some of the stars of the movies. If you are not a Star Wars fan, you might want to plan your days to visit Disney's Hollywood Studios on weekdays instead of weekends to avoid the crowds it draws.
I would love to help you plan more Joanne; however, I am limited in the amount of space that I have to answer. Please do not hesitate to ask if you have any further questions!