It's normal for mood to be affected by the weather, but for those of us with Bipolar, we're more at risk for that being a factor. It's true, sometimes there's no apparent triggers though for emotional difficulties. It's the nature of BP. Btw, my pdoc said that spring time tends to bring out hypomanic/manic states. This is true for me this year, and actually, I think I experience this pattern in general with the seasons. I feel more irritable-hypomanic on the rainy days, and just generally hypo on other days lately.

It's good that the dark thoughts you were experiencing have lifted and that the meds help, but those moods that come and go are hard to deal with. Sorry you're experiencing that.
If the mood swings get worse, there's the option of making a sooner appointment with your pdoc.