Do you find the time just before and during menstruation extremely tiring?  Are your energy levels at rock bottom and your usual motivation levels flagging? Are you looking for a way to exercise gently without adding to the overwhelming feelings of exhaustion? 
If you like the thought of a practice which leans more towards the restful and re-energising read on...
Ultimately it is about what intuitively feels right for YOU. 
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Practicing a Yoga sequence which is focused on helping to reduce the symptoms of PMS, such as cramping, low back ache and headaches can help us feel less crotchety! 
Exercises/ Asanas and breathing techniques that are helpful at this time focus on encouraging activation of the body's own natural relaxation response; which helps to relieve muscular tension and tightness, encouraging softness and release. Strong abdominal activation, harsh closed twists, headstands and full inversions have the potential to make you feel weaker, tighten the psoas muscles and strain the broad ligament which helps to support the uterus. These  combined with the increase in the hormone *Relaxin (which softens ligament and joints) after ovulation can lead to increased risk of strain or injury to the low back and sacrum. 
A practice that encourages a natural opening and flow, can over time help to release congestion in the pelvic bowl, reduce cramping, calm the nervous system, reduce headaches and lead to a more healthy cycle.  
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Even if you haven't got time to take it easy for the whole of your bleed doing it on the first day when you are bleeding more heavily may help you to re-energise and feel more rested. 
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'Many women turn more mentally inwards during menstruation and may notice that they become less sociable.
​Taking on challenging tasks may seem too much of an upheaval and rational thinking may be more difficult.
Menstruation is often a good time to slow down and reflect' 
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​I will be posting more on this soon.  
References *
https://www.wisegeek.com/what-is-relaxin.htm
​https:blissbabyyoga.com