I Can't Stop Sleeping On Weekends. I Haven't Been Out

I can't stop sleeping on weekends. I haven't been out anywhere - sleep fairly normal during the week and have been asleep for the past 16 hours waking up every few hours and am still so exhausted I can't even move and want to sleep more. This has been happening during the week to i feel so lathargic and tired when I have to get up. My vitamin b12 was low but I've already had 3 weekly injections and my iron is fine