R.J.
Yup...anything that messes with your hormones (from stress to nursing...and weaning does both!) can affect your period. If you're late and you're not testing positive, you're probably not pregnant...yet. Stopping nursing can frequently cause a short period of SUPER-FERTILE-WOMAN, though. So watch out. :)
You CAN have false negatives in home pregnancy tests. One of the most common causes is from dilute urine. So if you're really concerned, test right after you wake up.
R