Failure of periods to return after childbirth

Hi;
Was wondering if anyone would have any thoughts on why my periods have not returned after giving birth 11 weeks ago. I have already asked this question to one of the doctors here but forgot to say that I was bottle feeding not breast feeding. After birth bleeding was constant for 2 weeks then spotting for 2 weeks. I have done numerous pregnancy tests that have been negative. Am really starting to worry that something is seriously wrong with me.
Any thoughts would be great