I have googled this 90 different ways. I am super paranoid about everything...not that YOU are but I just worrry about it all. Anyway, apparently brown blood is old blood (sorry to be gross). I have stopped using tampons for the most part and this happens less. I also read that if this happens mid cycle it's most likely bleeding from ovulation.
I am not on the pill also, just incase you wondered.
I'll copy and paste what I found:
Whether you've always had regular menstrual cycles or never had a regular menstrual cycle in your life, the sudden appearance of mid-cycle spotting can be startling. Spotting is when a small amount of blood is passed, just enough to "spot" your underwear or toilet tissue. The blood spots can be many different colors, from brown to red to pinkish, so don't be too concerned about the color of mid-cycle spotting. Fortunately, mid-cycle spotting is usually nothing to worry about. Here is some information to better understand mid-cycle spotting:
- Changes in Hormonal Balance. All of the changes in a woman's menstrual cycle are due to alterations in the progesterone/estrogen balance. Changes in a woman's hormonal balance are especially common in perimenopause.
- Timing. If you have spotting that occurs around the time of ovulation (ten to fourteen days after the start of your last period), it is likely due to the increased estrogen in your system at this time. Spotting can also occur upon implantation of a fertilized egg into the uterine lining. Spotting that occurs several days to one week before one's menstrual cycle can be due to low levels of progesterone, which while not a problem in and of itself, can cause infertility or early miscarriage in the event that one is trying to become pregnant. Spotting that shows up at other times in your cycle is more likely to be abnormal spotting.
- Mittelschmerz. Mittelschmerz is German for "middle pain," and it is a word that describes the mid-cycle pain that can accompany ovulation. You will experience mittelschmerz on one side of your lower abdomen around the time of ovulation, from 10-14 days after the first day of your last period. If you experience mittelschmerz around the same time as spotting, you are almost certainly ovulating. Mittelschmerz is nothing to worry about; I see it as a convenient way of knowing exactly where I am in my cycle.