Bipolar disorder is a mood disorder that causes severe mood swings. Think of the "mood continuum" as having two ends, one end being mania and the other end being depression. Depression is characterized by profound sadness or apathy, loss of interest in activities the person used to enjoy, crying spells, irritability, insomnia, appetite changes, weight changes, lethargy, etc. People who are severely depressed may have thoughts of suicide. If your girlfriend tells you she does, then you need to get her to a doctor right away.
Mania, on the other hand, feels like the total opposite. Mania is characterized by extreme euphoria or irritability, tons of energy, having no desire to sleep, being very talkative, racing thoughts, a rush of productivity and/or creative ideas, reckless behavior such as speeding, drug use, indiscriminate sex, etc., poor judgment and spur of the moment decisions such as spending lots of money on things the person doesn't need, feeling "untouchable" like nothing can hurt them, etc. Mania can escalate into psychosis, meaning the person will start having hallucinations and/or delusions.
There is also such thing as a mixed episode in bipolar disorder, meaning a person has the symptoms of both mania and depression at the same time. That might seem counter-intuitive, but it's not, it's an extraordinarily unpleasant situation to be in for the person experiencing the disorder. It feels like having your mind and body torn in half, because in some respects you are so deeply depressed that you want to die, but in other ways you are bouncing off the walls, unable to contain yourself.
Mood swings do not happen "at the blink of an eye" like so many people wrongly suggest. Mood episodes have to, by definition, last for at least a week (mania) or two (depression) to count as actual mood episodes, according to the DSM-IV-TR. Manic episodes may last for several weeks, and depressive episodes may last for weeks or months on end. These are not Dr. Jekyll and Mr. Hyde mood shifts we are talking about, these are long-standing patterns of shifting mood over time.
I can't say whether or not your girlfriend's behavior over the summer was related to her bipolar disorder or not, nobody knows for sure but her. But it is totally possible that she did the things she did during a mood episode and was not really "aware" of her actions, or able to control them, because she was beholden to that mood state. Sometimes that happens to people who are not being treated for their disorder, even though it sounds like "just making excuses" to anyone else who has never experienced bipolar before.
That's not to say a person who has bipolar disorder has license to just do whatever they want because they have bipolar disorder, not at all. If your girlfriend has bipolar disorder then she has a responsibility to get herself treatment so that she does not have these severe mood episodes anymore. Medication can help a lot, it won't make the mood swings go away entirely but it does make them less frequent and less severe. Therapy can also be quite helpful in dealing with depressive episodes.
Here are some good, reputable websites with information about bipolar disorder:
NIMH: Bipolar Disorder
http://www.nimh.nih.gov/health/publications/bipolar-disorder/complete-index.shtml
Mayo Clinic: Bipolar Disorder
http://www.mayoclinic.com/health/bipolar-disorder/DS00356
WebMD: Bipolar Disorder Center: Symptoms, Types, Tests, and Treatments
http://www.webmd.com/bipolar-disorder/default.htm
NAMI: Bipolar Disorder
http://www.nami.org/Template.cfm?Section=By_Illness&Template=/TaggedPage/TaggedPageDisplay.cfm&TPLID=54&ContentID=23037
I hope some of that information is also helpful to you. The best way you can help your girlfriend is by educating yourself about the illness. That way you know what to expect and what you can do to help make this as easy as possible on both of you.
Good luck!