Periods shortening?

Sarah • I love my two beautiful children!
I recently had a baby 4 months ago, and we are TTC for our baby no.3  (keepin them close in age) anyways.. My periods are normally 7 days long on a 28 day cycle but my most recent one was only 4 days.. Not that I'm complaining, I just don't want it to interfere with us TTC. Anyone else have this problem