In my part of the world, we are headed into "cold and flu season." Therefore, the next phrase may be very important. You or one of your friends may be Feeling Under the Weather soon if it hasn't happened already.
This one actually does not have anything to do with the weather at all. (Except that you may think that the weather has contributed to your feeling this way.)
It just means that you feel ill and would rather be in bed. It can apply to almost any illness (not an injury) as long as it doesn't require you to go to the hospital. Though I have known a friend or two who didn't think that they were sick enough to be in the hospital because they were only a little under the weather.
If someone only has a small cold or is suffering from allergies, they might say that they only feeling a little under the weather. That would mean that they are basically okay, but don't feel quite right.
So, take your vitamins, get plenty of rest, and get your flu shot, so that you can be healthy while you observe others feeling under the weather.
Come back tomorrow, so that we can Strike While the Iron Is Hot.