What Causes Delayed Periods After Removal Of Mirnea Coil?

HELLO I HAVE A PROBLEM... I GOT THE MERENA REMOVED ON 24 FEB 2011 AND I HAD A SMALL BLEED FOR A FEW DAYS AND THEN 4/5 DAYS AFTER I GOT A VERY LIGHT PINK BLOODED PERIOD BUT I HAVE HAD NOTHING FROM THEN.. I HAVE DONE SEVERAL PREGNANCY TESTS AND THEYVE CAME BACK NEGATIVE :( IS THERE ANY WAY YOU CAN TELL ME WHATS GOING ON AS ME AND MY PARTNER ARE WANTING TO TRY FOR ANOTHER BABY (WE ALREADY HAVE A CHILD AGED 2)