What Causes Dizziness Followed By A Headache?

Hi, Today While In The Shower I Expierenced a very wierd feeling. i didnt quite know what was going on, like i was lost. and everything went pitch black but my eyes were open..i couldnt see a thing. for about a minute straight. this usually happens when i just stand up..but that wasnt the case this time. igot out and fell over i was very dizzy. followed by a terrible headache..ive also noticed reciently that there has been a popping sensation in the back of my head..can you help me determine whats wrong?