Why does my mood change -I'll be fine for a week or two then depression swoops in?

it's like a physical change comes over my body i can feel taking place - how can i find out if i'm bipolar or have clinical depression - i think i might be bipolar

Bi-polar is very real and very difficult to deal with. Bi-polar people typically have racing thoughts, poor impulse control, irritability, and insomnia. They can also have periods where they talk too much and too fast and are very hyper. This is during the manic phase of the illness, then they will dive into deep depressions. It would take a doctor to diagnose any mental conditions. If your mood swings are affecting your life I would see a doctor. If they are just annoying, it probably is hormones.

It could be generalized anxiety or isolated depression or bipolar disorder. No one can diagnose these unless you will have continuous symptoms for an least several months.
Best thing is have a psychiatric evaluation, they
know much better and provide a correct diagnosis so you will not be on unnecessory medications and all these episodes with stop.

Maybe you just have a lot of things on your mind
you could try to talk to a counselor to sort out your thoughts
or even before that, write down what's on your mind and read it back to yourself when you're done.
You're the only one who can really tell if what you're experiencing is serious, and if it is, you should see your doctor.
hope this helps:-)

Bipolar is just a stupid "disease". Don't believe it. It's just hormones.

It's probably just mood swings. It would be a good idea to see a counselor and be tested.

You need to be seen by a certified therapist and they can diagnose you. Good Luck!