23 weeks pregnant & feeling sad for no reason

So lately I’ve been feeling a lot more sick. I’ve been really nauseous which I’m used to because of my first trimester but lately I’ve been getting this sad feeling and this bad feeling I really can’t describe. I just all of sudden feel nervous and flustered and like all I want to do is cry and I don’t know why. It’s this overwhelming feeling of sadness and grief. I’m scared, I know women with really bad depression during pregnancy can have really bad postpartum. Is this normal? What should I do? I’m not sad because of my self esteem, for once. I don’t know why I’m sad. Someone reassure me.

UPDATE: I am bi polar. I was before I got pregnant & I also suffer from anxiety and depression.