Last week Tuesday I had a dark brown stain on my underwear, it went away, this Monday I had a pink tinge on the toilet paper when I wiped, it only happened once through out the day, yesterday it happened again but a little more red than pink, it happened twice during the day, today it happened again, I am having a little pain on my lower abdomen and I woke up with really bad heartburn, all pg tests are negative. What could this be?