You are correct that extreme exercising can have an effect on your cycle. However, this usually occurs with 3-4 hours per day of intense exercise. If your daily exercise is this intense, or if your body weight is below normal, then this could explain your very light periods. In addition, if your periods are seem to come and go, this would also be consistent with exercise being the cause.
Keep in mind that there are other causes of a light period. Once common cause of lighter periods is hormonal contraception
. If you starting taking birth control pills in the last few years, or if you had a Mirena IUD placed, then this would be the more likely explanation for your light periods.
I suggest that you schedule an appointment with your OBGYN. Bring with your a detailed calender of your last 3 or 4 cycles. Your doctor
will likely ask you how many tampons or pads you need to use in a given day. After your doctor gathers all of your information, then he or she will be able to tell if something is wrong. It is possible that your light periods are normal given your current state of health, or it may mean that you need to gain some weight.