I had my period twice within 2 weeks, it had stringy clots of blood and I had abdominal/cramp period pains what can this mean?

It's possible that is was a miscarriage, but not certain. It could be any of a variety of reasons, including just another period very early, or something like fibroids.
It is thought that many, many pregnancies spontaneously abort in the first days and few weeks, but statistics can't be tracked, and the embryo itself is too tiny to see, even though the uterine lining is building quickly.