A little late to be worrying about the weather now if you already have the tickets!
Thailand has summer-like weather just about top to bottom. Some times of the year it is a little hotter and a little more rainy. When it is rainy season the rain does not fall all over Thailand at the same time and usually the rain is over in an hour or two. When it is hot just adjust for it. Drink more liquids, wear a hat to keep the sun off of your face and walk in the shade when you can. Stop in stores with A/C to cool off. May will be low season, hotels will have discount prices and there won't be so many tourists around clogging up the works.
http://www.guidetothailand.com/thailand-…
Google give you lots of info on weather in Thailand. When you see maps of Thailand that say rain or cloudy weather keep in mind that it is usually only for a short time. Often the rain falls at night. For May the weather you get may depend on where you are, Thailand is bigger than you think! Don't waste all of your time at only beaches, try to see some of Northern Thailand too!
Good luck