Third trimester blues

I have no history of anxiety or depression so I have nothing to compare to. I just wake up feeling almost...homesick? Like a pit in my stomach every day. I have nothing to be upset about, I still go to work every day and interact with family and friends. But if I'm sitting with my mind unoccupied I feel so inexplicably sad! And also like I'll fall asleep :) anyone else experience this? This is my third pregnancy and I had moments like this with my other two but not every single moment of every day. So strange for me!