Did you get enough sleep last night? Are you getting less exercise? You said nothing seemed to make these feelings come on, but I just wanted to encourage you to make sure you are taking care of yourself physically as well as mentally. I recently found out I had low Vitamin D and I think taking Vitamin D also helps with my depression.

At any rate, I would give it a little time if your depression is not interfering with work or your daily life. It's normal for our moods to cycle every now and then even if we are mostly stable on meds. You know yourself better than anyone else, though, and if the intense feelings persist and concern you, I would make an appointment to see your pdoc sooner.

P.S. Changes in seasons can affect bipolar people (says my pdoc). I've felt more depressed lately, too.