I had this happen to me as well, at 3 years on T. However I never could find out why. Neither my endo nor my gyno could find a reason, then it stopped on its own.
For me, it started during August (2016) as just small amounts, but kept on every day for 3 months non-stop. One week a month during that time it was heavier bleeding (like a medium light/heavy period) then back to lighter spotting between. After those 3 months it stopped, then came back 3 or 4 weeks later, and I bled one week a month like my pre-T periods, for 3 months. Then it just stopped, in January 2017.
I talked to my endo several times during that second half of 2016, had my hormones checked, and saw the gyno in start of January the following year. I got no answer, they said I was healthy and my hormone levels were good (not too low or too high). I'm on Nebido so I only take my T once every 14th week, it's a different kind of injection.
It's been over a year now but I'm seriously still on edge about it and fear it happening again... my hysto really can't come too soon, for me. To stop my period-paranoia if not else.
I didn't find out why it happened, but I got a theory from my endo. That it can be so that the lining in the uterus/vagina can sometimes still build up when on T and then after a really long time (several years) "release" it like a period. I'm not saying that's what it is though, cause I really don't know.
