I Am Felling Weak, Thirsty All The Time And Tired

i am tired all the time, constantly thirsty but drinking loads, weeing more and feel quite weak, what could it be? lately in the past week i have been extremely tired and quite weak at times, my arms especially feel weak. i have also been constantly thirsty, even though i m drinking nearly 2 litres a day. this has made me urinate a lot more. i have also been getting a few headaches. i m 17 and work part time as well as going to college. i don t have early mornings as i m rarely ever up before 7:45, and i don t have late nights. what could these symptoms means?