Sometimes you don't feel very well, but you're not really ill. Here are some common expressions that you can use to describe general "aches and pains" and some useful "sympathetic" responses.
General aches and pains
"I feel a bit under the weather."
"I'm not feeling very well." / "I don't feel very well."
"I'm not feeling a hundred percent."
"I think I'm going down with a cold. I've got a sore throat."
(Other cold symptoms are a runny nose, or feeling a bit "achy".)
"I've got a slight headache."
Remember: pronounced "ake" as in "cake".
Other parts of your body which you can combine with "ache": toothache, stomach ache and backache. For all other parts of your body, say "I've got a pain in..."
"I'm not sleeping very well at the moment."
"I feel a little faint."
"I feel a bit dizzy." (when your head is spinning)
"I've got a nagging pain in my shoulder." (nagging = a pain that won't go away)
"I've got a splitting headache - I hope it's not a migraine."
You can use the verb "feel" in both the present simple and the present continuous to talk about your current health. The present continuous suggests a more temporary feeling, but there's not much difference between the two forms in meaning.
"I don't feel very well" = "I'm not feeling very well."
"I'm sorry to hear that."
"Actually, you don't look very well."
"You look a little pale."
"Maybe you're going down with something. There's a bug going around."
"Maybe you should go home and get some rest."
"Why don't you go home and have a lie-down."
"Is there anything I can do?"
"Shall I get you an aspirin?"
"I have a bit of a stomach bug."
"I think I've got a bit of a temperature."
"I have a touch of (the) flu." (Flu = influenza)
"I've got a nasty cough." (pronounced "coff")
With most mild illnesses, we use "a" as an article: a cold, a cough, a stomach bug.
However, we say "flu" or "the flu": "He's got flu" / "He's got the flu".
With serious illnesses, we generally don't use an article at all.
"She's got cancer", "He picked up Hepatitis", "Thousands of people used to die from cholera / typhoid".
More speaking help
Over on www.englishspeakingonline.com we've got some speaking exercises for you to practise your fluency. (You can listen to the conversations, record your part, then compare.)