You have one of two things going on: breakthrough ectopy, possibly (not absolutely) with some
AF and another treatment in your future (PACs, if they become more and more numerous, often indicate an imminent return to AF some time ahead)............or...................you are low om hydration or electrolytes....probably both. You continued to sweat for some time after you ceased your training, which means a continuation of electrolyte and blood volume loss. If this doesn't happen during your typical rest-of-the-day, and does happen frequently soon after exercise, what does that tell you?