If you know that you sometimes have blood from your anus after exercising, and the bleeding is in the appropriate part of your underwear, you may attribute stains of any size to that bleeding during the clean days. You should probably consult with your physician to determine why this is happening.
During the first three clean days, stains larger than a gris (about the size of an American dime or an Israeli shekel) are not usually attributed to other bleeding. But hemorrhoids are treated by some halachic decisors as a special case, to which a stain may be attributed even in these days. If your physician feels that the stains are due to hemorrhoids, you may rely on that halachic view. If not, please get back to us with more information.
On these three days, we advise wearing a white pantiliner during exercise, which would provide an additional basis for leniency. (Some halachic authorities treat pantiliners as a surface on which stains are treated leniently.) We also suggest staying hydrated!