Sounds like you're having a really tough pregnancy OP, I suffered through mine, it's just awful and especially hard when you have other children to take care of.
I think anyone who is feeling very ill, pregnant or not, should be able to use P&C spaces (as useful as they are, they are basically just a marketing device). However, I do think you have to be prepared to be challenged on it. I bet if you were feeling brighter you would have handled that woman's rudeness a lot better, but of course if you were feeling better, you would have had no need for the space in the first place! Plus, I don't know about you, but I was always a bloody hormonal mess in early pregnancy, which never helps!!
Do you have much support? The reason I ask is that I feel that with you being as ill as you are it would have been a lot safer to have a partner or friend with you to do the driving, fetch the water and generally be there to look out for you. I know that, as parents, we do get used to battling on, but in this instance it sounds like you possibly shouldn't have been driving. Pregnant + dehydrated + half starved from HG = high possibility of fainting/lightheadedness which could be dangerous when driving. I very reluctantly had to give up driving for a few weeks in my first pregnancy. It was a bloody nightmare logistically, but the safe thing to do.
I really hope you feel better very soon.