I'm confused about this?

I am trying to fall pregnant again and am confused by what my body is doing at the moment. My period always arrives like clockwork, except for this month when it is now 4 days late. I have done 3 pregnancy tests all of which gave a negative result. When I fell pregnant with my first baby I did a test 2 days after missing my period and it was very definitely positive. The only reason I can think to explain my late period is that I have just stopped feeding my son in the last 2 weeks and am thinking maybe that is interfering with my hormone levels and my cycle, but not sure. Has anyone had something similar happen to them??