I'm in the "if it's not really an option, don't ask" crowd.
I also give DS countdowns, and plenty of time to transition, because he doesn't transition well...actually, he's been doing a lot better lately, but for a while it was a big struggle until I figured out that was his problem, the transition.
I'm hoping you'll get a second wind too - I often find that if he's resistant to going somewhere, and I give him a choice, "you can walk to the car or I'll carry you" (as someone else said, not said meanly, kind of cheerily and matter of fact), and he collapses to the ground, I say "OK, I'll carry you!" and hoist him up over my shoulder upside down and bounce him to the car, and by about the third step he's laughing hysterically and has forgotten he "didn't want to". I feel your pain, I'm 25 weeks pregnant, but I do remember the earlier days when I wasn't feelign as good and it was a lot more difficult.
I also agree that unless there is an illness or some other emergency that should obviously keep you from going somewhere, that you can empathize, sympathize, talk gently and make the best of things that you can...I've done that a couple times (though I'm usually able to get him in the mood by being silly) and he's fine 2 minutes later, and we have a great time wherever we're going.
It's hard being pregnant with all its associated hormonal, physical, and mental changes, and having a very energetic, enthusiastic toddler to boot - I SO feel your pain!!!
If you're really not feeling well, and don't have the strength to playfully hoist him around (try some different holds- I like upside down over my shoudler and sideways like a football, they're easy on me, and DS is 32 pounds), you may just have to go with the flow a bit more until you're feelign better...I know that's probably not what you want to hear, sorry. I guess I would say that if you ask/talk about whatever it is a couple times and he's very resistant, I'd just drop it BEFORE you get frustrated,. I try to live by the 'ask once (or twice

), facilitate next' theory, where if you want to get something done and they resist, you *help* them do it right away, before you get upset and it becomes an issue....also known as "get off your butt" parenting...

Good luck with your iron studies!!
ETA: I'm totally with you on the bedtime thing, too -sometimes he's asleep in 10 minutes, sometimes it's an hour. My thing is if he's notout in 20 minutes, we get up and go into our room, he falls asleep, and then I go do what I want. Oh, and I've been going to sleep with him at 8pm every night this week so far, and feel great! SO, you may have hit on something, goign to bed with him for a while until you're feeling better.