Strange Mark On Platy

Hi,
I have a beautiful Red platy who is now pregnant. She has been fine and is still behaving fine but....this afternoon when the lights came on she now has a white streak on her tail. It does not appear like a growth or a  parasite of any sort and seems a perfectly natural part of her tail fin.

All of my water parameters are fine.  The only difference in my tank is that this morning I started dosing with plant fertilizer and liquid C02.

I dosed 8ml of Neutro T fert and 3ml of Neutro C02 both from Aqua Essentials.

I wonder whether the white part of her tail fin is happening because she is quite heavily pregnant??

As I say apart from that she is acting perfectly normal.

I am unable to take a photo at the moment.
Thanks