Only just seen this, but I Had this a couple of times, when I was in College and again when I was at Uni. Just take it easy, and don't get frustrated at yourself, it will get better in the end. And don't let anyone make you feel guilty for feeling rotten, CFS isn't a trivial little thing, the me association can offer support or advice if you need it.
Get plenty of sleep, and if you are doing something and you feel tired or achey, stop doing it and take a breather for a while. You don't need to give up cycling, I didn't give up kayaking when I was suffering from it at uni, at times it was the only thing got me up and out, and it gave me something to look forward to.
You might need to cut back though, maybe a couple of short gentle rides a week or something, the important thing is not to force it, take it very easy, and just enjoy getting out on the bike, rather than doing anything big and long and tough.
And get well soon 😉