I Am Seven Days Late On My Periods. My Baby

I am seven days late on my periods. My baby is nine months old. For last eight months after delivery was getting exactly after 28 29 days cycle. This time its already seven days over.
Did pregnancy test just now it came negative. Had unprotected intercourse too around one week before period date thinking its not fertile day.
I have period cramps but blood not coming.
Should i go for some other checkup or is it normal?