I cant see how dehydration would cause a heavy head?
I know that a hefty dose of MDMA can make you feel very sleepy and the rushes you experience can make your head feel uncomfortable. Take less or try adding some caffeine to the mix so it doesnt floor you as much.
Is it an uncomfortable feeling? When I start rolling my head feels "heavy" and spacy and warm, but in a really nice way. That's the best way I can explain it. Idk if that's what you mean?