I don't think that he isn't caring - he may well have bought the tickets for you as a treat, thinking that as you had been so ill and stuck in the house, that you would enjoy an evening out!
As for the clothes, quite frankly, most men don't take much notice of what women are wearing - he'd open the wardrobe door , see the clothes, and think that you'd be fine - the fact that they no longer fit, wouldn't enter his head!
You need to sit him down and just tell him what you have told us now, while there is still a chance that the tickets can be resold, or he can get a refund.
There is nothing worse than being somewhere that you don't want to be when you are not feeling well. My ex bought tickets for a show when I was at the sicky part of my pregnancy - he just couldn't understand that I did not want to be sitting in a cramped space , in the middle of the row, when all I wanted to do was vomit!
Talk to your OH, and also see if alternative therapies such as accupuncture, a massage or a different drug from your GP can help with the pain.