May 2018, raining in Pattaya.

Isn't it too early for raining? Climate changes?

https://youtu.be/0KaWEgR1v8w

The past week has been rather good weather wise around Dongtan beach vicinity. It rained heavily for one hour on Tuesday afternoon, and it failed to reach Dongtan beach on Thursday afternoon leaving behind a faint drizzle. From the beach I could follow the progress of the thunderstorms starting in the East around Satahip leading inland and westward to BangSaray, Jomtian. By the time it reached Dongtan the storm had already lost most of the precipitation.
Dongtan beach is now completely cordoned off for the benefit of swimmers, with alternate stretches of sand between chair concessions available for beach goers not wanting to patronise the chairs. The jet ski and boat businesses are confined to the extreme ends of the beach.
I couldn't help noticing the presence of a resident model posing in the latest thong by Vivienne Westwood. The sun was shining brightly on him.

No, traditionally Songkran marks the beginning of the rainy season.

If anything, the rainy season has been more late than usual the last several years. Actually, last year was quite good, but the year before that the rainy season came unusually late, which was horrible because then you had about 2 months of extremely hot water with no rain or clouds, making it almost unbearable.

I find as long as it rains every 2 or 3 days during the hot season, then it helps keep the weather drastically cooler.

Here is an annual precipitation graph for Bangkok. In general it's quite close to the more regional graphs with some months moving on a left or right shift, but even then the movement is marginal.
For instance, the bar for Bangkok shows quite a lot of rain in May. In Hua Hin however ~ not too far away from Bangkok ~ May is usually thought of as being rather dry. But all the other bars match up quite well.

https://preview.ibb.co/i6mCRT/averag...nd_bangkok.png

This one is for Phuket:

https://image.ibb.co/ke9XRT/phuket.png

This one is for Chiang Mai:

https://image.ibb.co/fNWwmT/chiang_mai.png