Pretty sure my girlfriend is depressed, what's the best I can do for her?

She's undiagnosed, but fits the bill for bipolar II. She's been depressed for 2 months or so (this is my first time going through this with her), but lately she's become distant and won't say I love you anymore. She acts like everything is okay, and it's impossible to get her to talk about it without angering her. I'm completely convinced she's worth every last drop of it, she's amazing, and I can handle this, I just don't know what to do. Any suggestions or insight or advice or what have you would be appreciated, thanks.