What does it mean if you passout randomly ?

Ok so I was at sixflags today & i walked up the stairs to qo on a water ride & i felt dizzy . I told my bf i needed to sit down then next thinq i knew he was holding me up . I had fell . Scrapped my knee & finger all up . I couldnt walk for about 10 min because i felt so weak . I had to wait untill i recieved water . I dont think i was dehydrated because i just ate breakfast & had OJ . This is only the 2nd time this has happened to me in my 18 years of living . The first time i jus had finished eatinq & i went to take a shower . I felt the same way in the shower & told my bf i need to sit down . He physically took me out the shower & my eyes started rollinq bck . I felt myself passing out so i tried to stay alert . I was shaking . Then I jus opened my eyes & i was laying on the floor . I dont know whats wrong or why this keeps happening . I will be qoing to a doctors .