What Causes Delayed Periods With Negative Pregnancy Rest?

Hi, after giving birth to my second child in Feb 2010, I started getting my periods on 13th March 2011. In April 2011, it came by 15th April and now its already 30th May and I have not got my preiods yet. I did a home pregnancy test 2 days back and and it was negative. I'm currently on the following medication: 75mg Euthyrox (1 everyday for hypothyroidism), Iron tablets (1 everyday), Vit D tablet (2 tablets per week). My monthy cycle usually averages at 35 days and I used to have irregular periods (sometimes a little more than 40 days) before marriage (6 years back)