One of your employees looks sick. You ask her why she came to work, and she says that she has a lot of work to do. You'd rather she go home so that she doesn't make other people sick. You say:
If you're feeling under the weather, by all means go home and get some rest.
You're feeling sick with a sinus infection. You were supposed to meet a friend for dinner tomorrow evening, but you don't think you're going to feel better by then. You're writing an email to explain why you can't make it. You write:
I've been having excruciating sinus headaches for the last few days.
cancel, email, health, pain, progressive aspect, sick, ungradable adjectives 
You're feeling sick with a sinus infection. You went to a doctor and got some medicine, but you don't feel better yet. You were supposed to meet a friend for dinner tomorrow evening, but you don't think you're going to feel better by then. So now you're writing an e-mail to cancel the dinner. You explain why:
I went to a doctor and got some antibiotics, but she said that it would take a few days for them to kick in.
explanation, health, medicine, prescription, sick 
You're out with a friend and your stomach started to feel upset. Your friend asks you about it, and you explain what's happening:
That burrito I ate didn't agree with me.
You're out with a friend, and she starts to look like she's uncomfortable. She's holding her stomach. You think that she might be sick.
What's wrong? Do you have an upset stomach?
When you "have an upset stomach", it means that your stomach feels strange. It feels like you might need to throw up or have diarrhea soon.
This is similar to "having a stomach ache" but a little different. A "stomach ache" sounds like you have some kind of pain there. An "upset stomach", on the other hand, doesn't mean that you have pain. It just means that the food in your stomach is going to need to come out soon in one way or another.
You can also say "(someone's) stomach is upset":
What's wrong? Is your stomach upset?
You're talking to a friend on the telephone. Her voice sounds deep and scratchy. It sounds like she's sick. You say:
You sound horrible!
Your friend is learning to play the guitar. He's frustrated because he doesn't think he's improving. But you do think he's improving. You say:
I can definitely tell you're getting better.
When you "can tell" something, it means that you know it by experiencing it:
I can tell that you're lying to me.
Could you tell I wanted to leave?
You can leave "that" out in casual conversation, but in formal speaking or in writing you should definitely include it:
I can definitely tell that you're improving.
When you use "definitely", it means that you're sure.
"Definitely" is often used together with "can tell":
I can definitely tell that you've lost some weight.
When you're "getting better", it means you're improving. "Get better" is a more casual version of "improve".
"Get better" also means to recover after a sickness or injury, by the way.
adverb, health, progressive aspect, recover, sick You're sneezing a lot. Your friend asks you what's wrong. You think you're sneezing because of your allergies. You say:
There must be a lot of pollen.
When you see evidence of something, or you have a strong reason to believe something, you can say "There must be ___". For example, when you hear music playing and people talking in the house next door to you, you can say:
There must be a party going on.
If your toilet doesn't flush correctly, you can say:
There must be something stuck in there.
Some English learners use the word "much" too much! In normal conversational English, "a lot of ___" is far more common than "much ___":
There was a lot of dust in the air.
My daughter always likes to put a lot of ketchup on her fries.
However, you can use "much" when you're saying "not much":
There's not much money left.
And it's good in the phrase "too much":
Sorry, I can't. I have way too much work to do here.
You can also use "much" for very, very formal positive sentences. When I hear a sentence like this, it reminds me of the Bible! It has that level of formality and age.
There was much rejoicing among the people.
"Many" is the same as "much". It sounds strange and old-fashioned in positive sentences, but normal in questions and negative sentences.
There were a lot of people there.
There aren't very many chairs left.
"Pollen" is the dust that flowers produce. A lot of people are allergic to pollen.
allergies, explanation, sick, sneezing 