Dehydrated

So went home early from work today felt dizzy n just really sick both my arms feel like I have no strength in them I'm 18 wks n 2 days. I have been drinking nothing but water for like 3 wks but the nurse said it sounded like I as dehydrated I just don't understand if all I'm drinking is water how I can b dehydrated she also said I may have a potassium deficiency just confused about the whole thing n I'm still feeling really bad wondering if I shouldn't go to er or just wait it out what do y'all think??