My pdoc told me that after a Bipolar depression you're vulnerable to another episode of depression for 8 weeks after returning to baseline. Maybe this is why you (and any of us) sometimes become depressed again even if it's after we feel better? Though I hope this isn't your case -maybe the low mood will change and you'll just go back to baseline, I hope this for you.

I wish you the best. What you're describing is actually starting to happen to me now, but it may be situational, not sure. Either way, depression is depression, and it really sucks.