trying to get PG !!

Hello i am a 24 year old healthy mother of a 4 year old boy (Julian) ...i am very worried because i had the iud for about 18 months i removed it July 14 09 and ever since have been trying to get pregnant... my period is always on time i get it every month on the 16th latest 18..today is dec 23 and i still havn't got it ..i do not feel any symptoms at all of being pregnant unlike my first pregnancy i felt fatal...i hacve the soar breasts as if i where to get my period but i havn't i have had 3 pregnancy tests in all and all show up Negative ..could it be something else ?? very worried