April/May is historically the hottest month throughout Thailand. There may be some local exceptions, but generally these 2 months are at the top for most/all regions. For whatever the causes, the temps globally are trending up, so no surprise it's hotter this year than last or 5-10-20 years back. This year is also an "El Nino" weather pattern year, which makes for hotter and drier conditions in SE Asia.
This an excellent and easy to use sight for climate data, averages, etc for nearly anywhere. Just input a country, province, state or city into the search ????.
https://www.timeanddate.com/weather/thailand/bangkok/climate