My brother is diagnosed with bipolar disorder and he's been on medication for it for the past 7 years. Bipolar can't be just casually diagnosed - as in, it's quite a serious disorder and only really obvious cyclical changes of mood over a period of more than six months, due to imabalance of brain chemicals, can be considered bipolar disorder.
Everyone feels down sometimes, and happy other times, but in bipolar, everything is excess. My brother really hates any reference to his disorder, but I think an example of his behaviour would help:
In the manic (feel-good) part of his cycle, there's a surge of dopamine and serotonin. Not only does this make him very happy and very vocal, he makes grandiose plans such as, "In five years, I'll make enough money to build a house!" or something like that. He sleeps less (he's up all night making plans), he has angry outbursts if someone points out the impracticality of his plans, he talks a lot about a lot of things to a lot of people, he starts huge projects like art or music classes that he thinks he has the energy to get through, and he picks quarrels very easily and becomes quite rude with random people. During this phase he can be quite frighteningly enthusiastic and loud-mouthed and wants nothing and no one standing in his way to greatness. He also starts believing in God and wants to visit temples. Normally, he is an agnostic, but now he starts thinking he sees signs of God or hears voices, etc. He's also apt to do impulsive things like call a random author up and tell them he wants to write a book in collaboration with them or something. He finds me lacking in ambition and tells me how beautiful and talented I am and I must do something about it, and gets angry if I don't nod along and take him seriously. His blood pressure is a lot higher than it should be.
In his depressed phase, he suddenly wakes up one day to see the futility of life. More often than not, this phase needs no trigger. It can be brought on by something as simple as a headline saying that someone murdered someone else, or that the market is crashing. Then he starts reviewing all the grand lists he made in his other phase and getting unhappy that he can never reach those goals. He compares himself to other people constantly, he cries all the time, and he makes up with people he was rude to before, often clinging to them as if they are his only friends. He tells everyone in the family he loves them and how great they are. He looks at pictures of our childhood and wants us to be little again. Often, he will burst into tears while he revisits our childhood, and says he wishes I was still 2 and he 7. Moreover, because we lived in Britain for three years of our childhood, he misses it a lot and wants to go back there. This is the point where he does something bad (but not suicidal) to himself if he is unsupervised, like drink himself into a stupor or try to starve himself, so he needs to be monitored and is usually put in hospital anyway because of fluctuations in blood pressure and heart rhythm.
When he is stable again, he seems not to remember anything about his manic phase and listens with wonder to stories of whatever he did. He knows what he is doing when he is depressed, but often he fails to reliase HOW far into depression he really went. He hates that I am "cold" and "analytical" about his condition, but honestly, I have seen him over the years and have a very doctor-like view of it, but only because I guess I'm trying to tell myself it's not really that big a deal.
Actually, because he is bipolar, I can't make out whether he is INFP, ENFP, INTP or ENTP. It's impossible to tell, because for the past few years he has swung between these types so often. When he was a child, he was very clearly INTP, but he's always been prone to violent moods and unfortunately it's impossible to make out anything about his type now.
I guess I wrote all this because I've been wanting to for a long time (it's my way of saying that I validate his disorder) but also to say that bipolar can manifest itself in such violent symptoms as to dirupt lives, too. There are milder versions of this (cyclothymia), and there are versions of only mania (I think it's called hypomania? Sorry, I do have a major in psychology but it's been a few years now and I forget easily) and only depression. I think if you have even a glimmer of this kind of behaviour, then you should be worried.