Hi,
According to your description the possibilities are:
-Changes in exercise, diet, sleep, etc. can alter the reproductive hormone levels and can lead to menstrual irregularity.
As you have mentioned that you have increased your exercise activity there is possibility that, it might have affected your reproductive hormone levels. And because of that fluctuation you may got the intermenstrual spotting. When the spotting mixed with
vaginal discharge, that can appear as brownish mucus discharge.
Other possibilities are local causes like cervical erosion, polyps etc. But as you have mentioned that, you have undergone clinical examination by the gynecologist and there was no specific findings. The first possibility should be considered more. Once your exercise schedule gets regularize, possibly this instrumental intermenstrual spotting may decrease and you may not need any specific treatment for this.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Sree Gouri S. R., OBGYN