i don't really get why i'm so tired..

i've really hurt my back today, i don't know how but its absolutley killing me.. i've currently got a hot water bottle on my back where it hurts but it's not really helping. i'm really tired as well.. i don't really get why i'm so tired because i slept for almost 8 hours last night which must be some kind of record for me on a college night, oh well! i will have to have an early night again tonight although i have to write my english commentary up again.. urgh!