28 weeks- the struggle has begun

I am 28 weeks pregnant with twins. I am also a middle school English teacher. I have already had to cut back on the hours I spend at work by no longer tutoring students after school, but lately even the regular school day is starting to kill me. This past week each day when I got home, my feet were swollen and my legs ached so badly I was in bed and not getting up by 8.

Now, I feel like I spent the whole day on my feet, and I just woke up!

I'm worried about how hard it is for me to do even basic tasks now for any length of time. I'm not ready to stop working yet, and I still have SO much to do before the babies come.

Is this normal? I feel like I shouldn't be this useless yet.

How do you cope with the aching and exhaustion? Advice please? Encouragement? Hope?