...maybe it's my own fault for trying to make it look easier than it is, so as not to freak anyone out...

'Kay, background: I am almost five months pregnant, and, while it hasn't been a picnic, I've definitely had it a bit easier than a lot of my girlfriends. I've been learning a LOT about pregnancy that I did not know, and some stuff that I'd hear and just not understand, because I guess some of this stuff really is the sort of thing you've got to live to "get".
That said, before I became pregnant, we were having a hard time getting there, and I'd had to distance myself from several friends who would NOT STOP COMPLAINING about being pregnant, about having a kid, about having another kid, about staying home, about not wanting to work (same person!

) etc. etc. Even at the time, I knew that pregnancy was not a big bowl of sweet juicy cherries, but, really? NOTHING but complaints? Do you not know how lucky you are, and how I would trade places with you in a second? All of which is why I really try responding to a "How are you feeling" with a cheery "Just fine!" and focusing on a positive thing that had happened, or that I was feeling, instead of giving the asker a blow by blow of my latest complaint/sick feeling, etc.
Now, I am afraid that this "Pollyanna Policy" is coming back to bite me in the rear, because two of our friends just do NOT understand why I cannot be their DD tonight. To their thinking, it's easy: I won't be drinking. They will. (And so will DH, but DH has never, and does not now have, a problem with being responsible and careful and driving himself/me safely home. I've never felt comfortable driving at night, even before the baby.) We live near each other. DH and I have the roomier car. From the exchange we had with them, it became quite clear that they think that I am suddenly playing the role of "delicate little flower" just because I don't feel like doing them a favor.
The problem is that while we've been planning tonight's event for quite some time, an emergency arose in the past week which requires me to be up and out early every morning for the past few days and the next few as well. I've been taking naps in the afternoon, and eating extremely healthy, staying hydrated, etc, but for the past few nights, after just STAYING IN, I've been fading at eight p.m. I'm trying to conserve my energy even more today, and I'm hoping that I can calm down enough to take a longer nap this afternoon, but I can't guarantee our friends that the "Designated Driver" won't be having to head for home and bed earlier than they'd like.
So I feel like I'm either being "selfish" now, or will be a "party-pooper" later, and neither hat is one I'm used to having slapped on my head. A (probably rather irrational) part of me wants to bow out altogether and let DH drive these two while I stay home with my Netflix and hot cocoa, even though I've been looking forward to this event for months, and was so thrilled when my sicky-feeling started to subside, so that I could enjoy it even more.
Has anyone else experienced having to suddenly fess up to the truth of the matter at an inopportune moment, and has some tips or advice?
Should I just stay home and let the non-gestators whoop it up as late as they want? I'm afraid I'm not thinking too clearly right now; I'd really appreciate an objective look-see.
ETA: Just want to mention that this is not a night of bar-hopping and clubbing that I'm horning in on. It's part of a several-times-yearly series of dinner parties that we all take turns hosting, and this particular evening has been in the works for longer than I've been pregnant.