Why i am so tired lately?

i have a 5 month old baby and have been feeling great since i had him, even though i lost alot of bloody through a csection and have been on iron pills to help. i never had an problem with feeling tired, but this whole week i have felt so tired.last night i slep 7 hours, woke up and then went back to bed for 2hours and letr went back to bed for another 2hours, i still feel tired. for the last week i feel like this, i don't think i am pregnant again because i had my period last saturday, it lasted 3days and i didn't have sex for 2weeks befor that. i also try to excise as much as i can, i always take the baby for a walk and then the dog for her own lil walk to each day. what could be making me so tired?