It might be due to dehydration if ur doing heavy exercise . The first thing you need to do is drink plenty of fluids with salt and sugar in it . The next thing it might be due to hypokalaemia which is due to low pottasium levels , so take drinks which have higher pottasium levels. So this are the common problems. Some uncommon reasons may be due to diabetes pritonitis (inflammation of peritoneum),if u have excessive urine disharge these days along with visual impairment and high bloodpressure , then its better you go for diabetes test. If you have loss of appetite, nausea, vommiting then you go for intestinal check up. But the next two reasons are uncommon , so most probabaly it might be due to dehydration. If it doesnt solve with excessive fluid intake .Then go for the rest of the tests