I haven't had much of an appetite for the last few days, and I don't feel very energetic either.
I haven't felt like eating the last several days, so I'm feeling kind of lethargic as well.
この場合、次のような言い方ができます。
ーI haven't had much of an appetite for the last few days, and I don't feel very energetic either.
「ここ数日食欲がなくて、あまり元気もない」
appetite で「食欲」
don't feel very energetic で「あんまり元気がない」
ーI haven't felt like eating the last several days, so I'm feeling kind of lethargic as well.
「ここ数日、食べる気がしないので、ちょっとだるさも感じる」
to feel lethargic で「だるく感じる・力が出ない」
ご参考まで！
カナダ 
