A.K.
Let's see, you already have 3 kids, two are 4 year old boys, you're pregnant, and tired. I'd be shocked if you weren't!!! I got sympathy tired just reading your message.
Sorry, I don't mean to joke, you are obviously tired to the point it concerns you.
I'm not a health provider, but I'll make a couple of suggestions. First of all, could you be suffering from depression? Be honest.
If that is not your issue, are you hydrating? are you eating high energy foods throughout the day? are you getting rest, not necessarily sleep throughout the day?
Not all pregnancies are alike. You just may be more tired this time, you are 37. I had my first child when I was 31 and never felt younger. I had my second when I was 36 and never felt older and I'm 53 now. Also, you may just be this tired now, your whole pregnancy may not be this way. It may be like morning sickness that passes after a few weeks.
Take care of yourself, the next time you visit your dr, talk about it. Maybe you could get a different vitamin (you may want to consider splitting the pill and taking half in the am and half in the pm.)
Good luck and take care of yourself!