Are you drinking plenty of water with the kratom?
Outside of that, Fairly normal for some people to be sensitive to kratom. If 2 caps gets you where you wanna be then don't take more then 2 caps.
Could be that strain sure some are more potent then others, some are enhanced with extracts some are enhanced with other drugs like ODST or Tianeptine or even Toradol, But thats prolly rare these days but i dunno shitty kratom needs a boost to sell somehow.