For implantation bleeding you need to be
pregnant
So if you got a period at the right time this is your period flow
You are not pregnant
the colour does not matter
letrozole does not reduce the flow
Exercise and reduce weight if you are overweight that could be the reason of a scanty flow
get your
thyroid levels checked
If you have already taken letrozole for 7 cycles you need investigations to understand the reason your inability to conceive rather than continue with the same treatment