Lost 31LBS in 9 months for no apparent reason, muscles seem to have wasted, always feel tired what could it be?

As the question says I have lost 31lbs in the last 9 months. recently at a rate of 1lb a day. Nothing in my lifestyle has changed. I seem to have lost a lot of muscle but not much fat. I'm a little worried what could it be?
Lost 31LBS in 9 months for no apparent reason, muscles seem to have wasted, always feel tired what could it be?
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