Nirav promises to finish up his series explaining some of the nuances of the many words you can use to talk about one's condition, especially some of the words using 気, but in the meantime, I'm gonna toss in a few more feelings phrases "to help make your 言い回し more 日本人ぽい."
One thing that was really difficult for me, when I first came to Japan, was explaining to people how I felt, especially if how I felt was anything other than "元気." Nirav will get into this more in his post, but really, the only thing I was equipped with, in terms of saying, "I don't feel good" was "気分悪い."
However, telling people that I was 気分悪い almost always had the effect of making them think that I was sick, and giving me more attention when most of the time, what I wanted to let them know was that I was in the mood to be left alone.
Thanks to Brett, I now know that I can simply explain that "the bug's whereabouts are bad" but what about getting more specific about why you're not in a good mood?
Luckily, if you're in Japan long enough to form close relationships, especially romantic ones, you'll learn how to express all kinds of moods. And if you want to know what to say without having to get in a bunch of fights with a significant other, well, that's why The Daily Yo-ji is here.
to be/get frustrated; to feel frustration
Do NOT trust Rikai-chan on this one. I've NEVER heard this used to mean "getting nervous." See the next point on the list for that. Use this to express frustration, being fed up, and having things get on your nerves.
to be nervous; to feel nervous tension
ドキドキ is the sound of a heart beat, and when you get nervous, your heart starts beating faster. These are great words to use about anything that makes you nervous: having to give a speech, trying something for the first time, when the pressures on you to do or say the right thing, getting on a roller coaster, petting a lion, etc.
MOST of the time, you can use these two interchangeably. When I was meeting my girlfriend's family for the first time, I could (and did) use BOTH 緊張する and ドキドキする。
However, when you're with a boyfriend or girlfriend, you might feel 緊張, but you'd be better off telling them that you are ドキドキしている. 緊張 might be interpreted as, "You make me tense." ドキドキ is like, "You give me butterflies," or "You make my heart beat faster."
to feel sad; to be upset; to be gloomy
This is good for when you're down about something, and can't (or don't want to) explain why. When nothing's wrong, but you're still upset, or when a bunch of little things have combined to make you feel like EVERYTHING is depressing, that's もやもや.
to get upset; to reach the breaking point; to be fussy;
to throw a tantrum
Again, leave the Rikai-chan translation alone on this one. It'll just confuse you (to become perverse? inferiority complex? what?). It does have some other applications, but here's the easiest way to think of it: a child throwing a temper tantrum is いじけている. You can use it about yourself or others if it's a situation where the person in question is so upset that they can't do anything but be upset, or if they're making a show of how upset they are, like a crying child. I don't recommend using it, because I associate it with things like being わがまま (making a scene when you don't get your way) and the idea of 切れる emotionally where you just SNAP, but as others have pointed out, it also has the nuance of "being fussy," which, just like in English, can sometimes be thought of as endearing in small doses. And one more fun note about いじける： You can call people who are overdoing it, "いじけ虫." But watch out. If they really are pissed, you might make it worse!
After I posted this, Brett had trouble looking up いじける、which lead to long conversations with native Japanese speakers about いじける (it's 標準語, so it wasn't one of those "DAMN YOU, SAGA-BEN[ben = dialect]!" moments), and then today we got a comment from reader mico saying some of the same things. Bolded phrases above have been added to reflect where we needed to make changes, and a few sentences that were incorrect have been removed. Thanks for the input!