What Causes Inconsistencies In The Blood Content During Menstruation?

Hello
I am 39 and get regular periods every 29-30 days. When my period starts I always know as I get periods pains and I always have dark blood at the start. For the last 2 months about 2 days before my period is due when I go and have a bowel movement ( with straining) I have had bright red blood come out of my vagina( checked it was my vagina and not anus). I do not get constipated that often and am not pregnant and eat a healthy diet. The red blood from my vagina then lasts for about 2 days( very light) then I get my period pains and the usual darker blood starts flowing and I then use tampons.
What would cause the lighter red blood 2 days before my period with straining with a bowel movement? I had a smear 6 months ago which was ok and my cervix looked normal. Is the red blood just caused by straining and my period being so close? When it first happened I thought it was from my anus, piles, but I checked and I do not have piles and it was 100% from the vagina.
Thanks for your time and help