Yours isn't even growing slowly. Every second week another couple tells you they'll be moving out of town. The Treasurer tells you after service that the giving has been slipping over the last few months because several key members seem to be withholding their tithes for some reason or another. As your people left the church yesterday they didn't seem to notice that you had preached. That this isn't the real thing.
Most pastors have.
But the question isn't why pastors feel this way on some Mondays. Even through a string of Blue Mondays. I don't think so. I think it has something to do with The Call. Those who do ministry for fun, or money, or a nice career, or because their grandmother prayed them into it, don't make it through Blue Mondays.