Missing an Optifast shake?
I wouldn't worry about missing the shakes, just keep going and trying to get the 4 in for the rest of the time. As for the weight gain, could be water retention, TOM, and your system wondering what you are up to. Which will happen a lot over the next while. I wouldn't worry about it.
The exhaustion is from not "eating" enough, having the late night catch up, and you might be a little dehydrated, if you aren't getting the liquids you are used to getting. That and the Day 6 blues ;o)
You'll do fine with the rest of your opti.
Cathy