I think it is definitely worth talking to your GP and your midwife about; it's possible it is hormones, but it could be something deeper than that, and your manager certainly should not patronise you by brushing you off with "Oh, it's just hormones dear". I think I would have murdered somebody if they had said that to me.
As you well know by now there are peaks and troughs; some days you wake and feel that you are the luckiest, happiest person alive, and other days it seems frightening that everything is changging, your clothes don't fit, you're tired, you don't want to go out, you're sick of staying in, etc etc. If it is any consolation I was due 3rd April and was working until 28th March, and over the Christmas before I was due, I felt awful! I was so down, my back was killing me to the point I could neither sit nor walk, and I thought there was no chance I would make it to the 28th March, however, it seemed it was just a phase and I managed to make it the whole way (lazy baby didn't arrive til 13th April anyway lol).
One thing to be aware of is if your GP does decide to sign you off and he/she believes that your depression is pregnancy related then your workplace are entitled to begin your maternity early. If your GP thinks that it is not PG related (as happened to me, I had a chest infection), they can sign you off and it will not affect your mat leave, no matter when you go sick.
It might be worth approaching your manager again, or their line manager, telling them you are not happy at being brushed off and to look into avenues where you might be able to work from home, decrease your hours, etc.
All the best onlyme26 xx