Why am I tired all the time? I can't force myself to do anything?

I don't know why this happened but suddenly I've started to feel tired all the time. I'm not depressed but I don't know what it is. I usually exercise three-four times in a week but I haven't done it in a month because I feel like my body doesn't have energy to do it. I used to get up early in the morning and enjoy a morning walk in the sun to the gym but it seems so heavy now. I can't even bring myself to clean my room without having to lay down and rest, I rest and sleep all the time!! It's been like this for almost two months. What is wrong with me?
Why am I tired all the time? I can't force myself to do anything?
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