I just finished my menstrual cycle and my legs feel so weak. is it related?

Check for anemia: The most important thing is to be sure that you are not anemic i.e. have too low a blood count due to heavy flow. Low blood count and low iron can be easily checked on blood work by your doctor. Both the fluid and blood loss can give you a sense of weakness in the legs.

Period: If your periods are heavy, you could be anemic which in turn may give you weak legs.

It's possible: The menstrual cycle can be stressful, and can lead to physical ailments on occasion. If your period was very heavy, you may even be slightly anemic, which could cause weakness. If this happens often to you after your period, it certainly may be related, otherwise, see your doctor for other causes. Good luck.