Alright, I just had one week of complete lazyness because of fever ( I can't complain, I really enjoyed sleeping without feeling guilty!!).
Being sick for a week gave me a lot of time to think about different ways to tell that you're not feeling great. I'll go from the most general examples to the more precise ones :) Enjoy!
"Jeg er ikke god/bra i dag." (Lit:" I'm not good today")
"Jeg føler meg ikke helt bra." (Lit: "I'm not feeling totally well")
"Formen er ikke på topp i det siste." (Lit: "The shape isn't at it's peak lately." You can also use this expression when talking about how you're doing in sports.)
"Jeg er ikke helt pigg." (Lit: I'm not totally well. The word "pigg" usually means "thorn", but not in this expression. You can translate it by the Norwegian word "opplagt meaning "fresh, awake, well, with energy".).
"Jeg føler meg ikke i form." (I'm not feeling in shape. Can also be used about sports.)
"Jeg er skikkelig dårlig/uggen!" (Lit: I'm really unwell.)
Your friends might ask you:
"Hva feiler det deg?" (Lit: "What's wrong with you?" Can be negative in other contexts)
"Hvordan syk er du?" (Lit: "How sick are you?" Real translation: "What kind of sick are you?")
"Brygger du på noe, tror du?" (Lit: "Are you brewing on something?" Better said: "Are you about to get real sick?"
This is what you can answer:
"Jeg har feber og er kvalm." (Lit: I have a fever and I'm nauseous."
"Jeg er forkjølet, tror jeg." (Lit: "I am "coldened", I think. Better translation: I have a cold, I think")
"Jeg har vondt i halsen og er tett i nesen." (Lit: I have a pain in the throat and am congested in my nose." Better translation: "My throat is hurting and my nose is congested")
"Jeg er dårlig/uggen i magen." (Lit: "I'm unwell in my stomach", usually another way of saying "jeg har diaré")
"Kanskje er det influensa?" (Lit: "Maybe it's a flu?")
"Det er sikkert/nok et virus som går." (Lit: "It's probably a virus that goes." Better translation: "It's pobably a virus that's circulating."
"Jeg har hodepine og er ekstremt sliten." (Lit: "I have a headache and am extremely exhausted.")
And, the final touch:
"Jeg håper det ikke er noe alvorlig!" (Lit: "I hope it's nothing serious!")
"Jeg håper det går over fort." (Lit: "I hope it goes over quickly." Better translation: "I hope it finishes quickly.")
"Jeg trenger nok bare litt fred og ro." (Lit: "I probably only need some peace and calm.")
And, of course, if you don't have anybody around you to say this, then give yourself a pet on the shoulder and say to yourself:
GOD BEDRING!! ;) (Lit: "Good convalescence!")