Hello
In fact, at 39, you can most definitely begin the perimenopausal transition..
however, you are having your hormones tested, so they can let you know for sure.. If you are, it's NOT uncommon for you cycle to begin to change, one change is definitely the length of the cycle.. As you probably know, with the onset of one's period, estrogen levels drop and are at their lowest during the period, that said, with perimenopause, there is what I think of as a first initial big drop in estrogen, well... since estrogen drops, it could cause bleeding.. now, I am no doctor, but have been tracking my cycles from the age of about 39 up until now, I am 51.. I began to see PM changes around 39 and then by 42 I really began to feel the change because that is when my hormones began to get as I like to say, "shifty" :)
one minute I felt ok, the next... bam.. headache or mood swings..
same goes with my period.. started out at 5 days.. then shortened to about 3 and then back to 5 or 7... now, at 51.. while I longer ovulate, I do have anovulatory cycles that run for about 5 to 7 days with the overall cycles having shortened from 32 days to now 26...
when I first began to experience PM, I definitely thought it was my thyroid, turns out , it wasn't.. hormone fluctuations can definitely mimic thyroid conditions.. so it's good you are getting your hormones checked..