Hi Beauty,
I feel your pain! I am only 17 weeks pregnant but have not had a lot of energy lately myself for some reason. The thought of exercising does not make me nauseous but sometimes it is hard to get up the gumption to just do it. The only thing I can suggest is that you break up your workouts into smaller chunks. Instead of feeling like you have to walk for a full 30-40 minutes at one time, just do 10 minutes at a time a few times a day. Everything you do counts. I usually start my workout saying to myself that I will at least do 10 minutes and then if I am too tired, I will stop. Usually though, I end up going through a full 45-60 minute workout. The exercise generally revs me up and rids me of my energy slump at least for a while. Listen to your body though and if you are feeling truly too tired or are experiencing any pain, then stop. I got on my treadmill yesterday all set for a good walking stint but about 8 minutes into the exercise, I started getting lower abdominal pain. My ligaments are beginning to stretch and cause discomfort and they didn't seem to appreciate the exercise yesterday so I stopped. I will try again today. Good luck and just do what you can.