Confused about period?

My cycle has been pretty regular the last 3 years, with a day or two difference in length. Last week I thought I was starting my period but for the last 6 days it’s been like 3 drops of red blood per day and then a few wipes of brown discharge. Not enough for tampons or pads. pregnancy tests are negative. Why am I not getting a regular period? I usually only spot for 1 or 2 days and then get full flow for 5-6 days. Tomorrow is day 7 of intermittent brown spotting. Any ideas?