What Is Implantation Bleeding?
Unlikely to be implantation spotting.
Detailed Answer:
Hello,
The bleeding you described is unlikely to be uterine implantation spotting. When this happens (and it doesn't always happen), it is usually light spotting, and happens around a week after fertilization.
The bleeding and cramping you are having is more likely to be an early period from an anovulatory cycle. This is a cycle where you don't produce a mature egg. When this happens, periods are often atypical, coming earlier, later, being shorter or longer, and sometimes the flow is different.
Anovulatory cycles are not uncommon in the teen years and occasionally in the early 20s.
If the cramping becomes more severe pain, then go in to be seen to make sure it is not an ectopic pregnancy, but this is less likely.
Is there a reason you want to conceive when you are 18? Because there are more risks, to you and the baby, that are lower just a few years from now.