It is PROBABLY that your medication isn't working anymore!!! Many times we become accommodated to our medication -- we get used to it. Then it just stops working, and we have to have it changed to another. This sounds like what's happening to you!!! And it's dangerous.
Talk to your doctor VERY SOON. If it ever happens again, and you notice that you're slipping lower and lower, call your doctor immediately. You don't want to get to this very low point again! Too many people dismiss depression as just something that can be 'fixed' easily. That's not true.
So please call your doctor as soon as you can, okay? And let us know what happens. My prayers are with you. Take care & God bless. Hugs, Lee