One of my readers asked about exercise while menstruating, which is really a good question for all you women out there, so I decided I would answer this question first.
The answer to this question ultimately has to come from you. Some women have really easy menstrual periods and it's not a big deal for them to continue their normal workouts. Others, like me, tend to have horrible periods with everything you DON'T want; bad cramps and headaches, backache, bloating, nausea, fatigue, mood swings... well, you get the point. The trick with those periods is that you can usually workout and it will actually help relieve the symptoms somewhat. The most important thing for you to do during your period is to listen to your body. I'm not saying to completely take it off, but if you experience any worsening of your symptoms when you try a workout (usually this happens during the first couple days), stop. However, if you're just using your period as an excuse, knock that off! What is generally recommended is a slightly 'lighter' workout. Basically, just listen to your body and what it's telling you.
If you are a regular yogi, you probably already know this from your classes, but you shouldn't do any inverted poses (where your head is lower than your hips). There are some really good yoga poses and 'workouts' out there that can help relieve the symptoms even more. in particular; half-moon pose, supported child's pose, and reclining bound angle pose. If you are interested in the yoga route when you are menstruating, I highly recommend you look at Moving Towards Balance: 8 weeks of yoga with Rodney yee (in the back of the book there is a 'moon practice' that is really good. I also recommend you look at The Woman's Book of Yoga and Health.
I often find that light cardio or a lighter bodyweight or dumbbell workout helps relieve some of the discomfort and makes me feel better. The general consensus is that you should try to continue your workouts, they will help combat the fatigue - as long as you don't overdo it - and will help relieve other symptoms as well.
The main idea is that you listen to your body. Not the excuses your mind gives you, but what your body is actually saying. Try your workout and pay attention to how it's going. If you're feeling worse, stop. If you're feeling better, finish out your workout. Just like your body, your period is individual and not like everyone else's, so pay attention to yourself and do what your body is telling you to.