L.,
Congratulations on becoming pregnant!
You are unique. Anything could happen. :)
I couldn't wear my everyday pants after 3 weeks.
I was TIRED the whole time, but really wiped the first 4.5 months. It was, coincidentally, the period that my body decided food wasn't such a great idea. I ate anyway, but I used to *LOVE* food.
For the Yoga, do it anyway. Just try. you will feel better as the days go on - even if you only get to a sitting position in front of the TV - you are still benefiting - from the music (baby) and attention to yourself.
Once you start nesting, you'll find more enthusiasm. Just don't plan on painting the baby's room the month before you're due.
Are you taking prenatal vitamins? Are you spreading them out during the day (instead of taking them all at once?) Are you eating anyway? Are you drinking all your water (remember, you have to have enough to flush YOUR body, your baby's bod, AND keep you hydrated now that it is summer.
Good luck, and let me know how you do with this. :)
M.