Yes, this is actually entirely usual. I really suffered in the first trimester and a little in the second. I had no problems with first pg but this one has been so different. Far more of a struggle with my hormones.
Do tell your MW and if it gets too bad go and see you GP. I had some very dark thoughts at one point and posted on here. It is called antenatal depression and although PND has a lot of press this doesn't get so much.
Also, although it didn't seem like it helped, exercise (like your walk) is probably one of the best things to do.
The other things are: don't let your thoughts get out of control (e.g. ending it!) and remember it's not you it's just this thing you have at the moment, don't feel guilty that you're feeling low and not jumping for joy at this miraculous pregnancy (as some lead you to believe you should be!) and look after yourself. I found it helped to talk to DH and my close family and friends. Everyone was so lovely to me and it just felt better that they knew.
I would share my packet of hobnobs with you but I polished the whole packet off an hour ago (and now feel distinctly ill).
I hope you feel better soon.
Oh and where are you to get sunshine? It's raining here...