I had a different chemo regime to you. I had 3 weekly chemo, 3 x FEC and then 3 x Docetaxel. My periods continued up to the 4th chemo (first Docetaxel). My oncologist was surprised they continued so long into chemo but kept saying they would stop soon! I was then told that I would likely stay in menopause because of my age (47 at diagnosis) but 18 months later my periods returned. My oncologist said this was a common time for ovaries to recover from chemo effects but was surprised that mine did because of my age.
As I had hormone positive cancer (and was 49 by this time) I then had my ovaries removed so I could change from Tamoxifen to Femara which gives a bit better protection against recurrence.
I guess we are all a bit different and sometimes our bodies put up a pretty good fight against the side effects of treatment. Yours is doing this too. I am sure the chemo is still doing its job getting rid of any stray cancer cells though. xx