Could you be pregnant if you bled one tampon full and that was it and you didn't have a period and are now having pregnancy symptoms?

Yes, it is possible. Some women do not know that they are pregnant because they may have light spotting or bleeding that they disregard as a period. If this little bleeding is abnormal for you, take a home test. In the meantime, if the bleeding begins again, don't use tampons until you rule out the possibility of pregnancy.