Tamoxifen and periods

I have recently finished my chemo treatment back in November then commenced Tamoxifen soon after. I have been lucky enough to have had no side effects up until now. I have just got my period after not having one since August. Nine days later and it is showing no signs of going. Have others had this problem after chemo has finished and starting tamoxifen? My oncologist said this is normal but im starting to get a little concerned. Maybe the tamoxifen isnt working ??  Or am i just getting paranoid.