Hi there,
First of all I must say, I am not too overly stressed about this cause it might just be me, but thought that I would put it out there and see what you think....

So the average period should be 5-7 days yeh? Mine is only averaging to be about 3 days and maybe 1/2 a day of spotting if I am lucky.
I have been off the pill for 4 months now and my period has gradually been getting lighter and shorter:
Cycle #1 - heavy to medium flow lasting about 4 days
#2 - 2 days light, 1 medium, 2 light and 1 spotting
#3 - 1 medium, 2 light and 1 spotting
#4 - 2 days light, 1 day light/spotting.

Im not quite finished on #4 but I dare say its going to be tapering off today..

So could the light/short flow been I don't have a thick enough lining for an egg to implant or has this got nothing to do with it?

With my successful cycle, I had 3 days light and 3 days spotting..