donated blood, now extremely exhausted, how long will this last?!?!

i gave blood yesterday about 4pm, and felt fine the rest of the day pretty much, but now this day after, i feel extremely tired.. to the point where i had to skip a few of my class today. it really really stinks, no one else seems to be bothered. but my friend told me ill prob feel like this for a whole week! and my sister said it takes a month for you to be able to exercise the same way. is this true?!?