I'm sorry you are having a rough time.
There is a spectrum of bipolar type problems or mood experiences. Feeling happy is generally normal. Feeling suddenly happy in the middle of a serious depressive episode that has been going on for weeks is not typical of unipolar depression, and deserves a closer look.

Here are some links to resources on Bipolar disorder that may help you compare your experiences with what is commonly classified as Bipolar (or the broader spectrum of mood disorders).
http://psychcentral.com/disorders/bipolar/
http://psychcentral.com/resources/Bipolar/
http://psychcentral.com/quizzes/bipolarquiz.htm

Whether your problem is Bipolar or not, it is a good thing to have it checked out.

But be careful: if you think you have any mood instability symptoms, it is important to rule out the possibility of Bipolar before going onto antidepressant medication. Antidepressants on their own can destabilise a person with latent bipolar disorder in a very serious way, and can lead to mania and suicide if not monitored very closely.

http://www.justiceseekers.com/files/NLPP00000/072.pdf for a detailed discussion on antidepressants, mania, suicide and other issues of instability.
If you have good reason to suspect Bipolar, it is a good idea to get stable on a mood stabiliser before beginning an antidepressant.

Hope that helps.