If your egg got fertilized but didn't implant what does this mean?

Nature: In a woman's natural reproductive cycle approximately 60 to 80% of the time, an egg and sperm will combine forming an embryo but only 15 to 20% of these embryos will implant each cycle. These percentage of incidence will occur in each cycle.

Not a true pregnancy: The uterine environment may be hostile, such as with an iud, or when there are polyps or fibroids in the uterus, or hormonal balance may not be right, such as with a low Progesterone or the uterus does not allow for implantation. There is a positive pregnancy test(chemical evidence for fertilization) but no implantation or further development occur. Testing should be done by your ob-gyn.

Many reasons.: I presume you know that fertilization occurred because you are undergoing in vitro fertilization. There is no way of knowing if/when fertilization occurred in a spontaneous conception, unless it results in pregnancy. The next necessary step after fertilization is implantation of the zygote into the endometrium. Many conditions can prevent this: plan b, infection, fibroids, polyps, smoking, anomaly.

Not pregnant: This likely happens frequently to women, but would go undetected as the menstrual period would come on time and a pregnancy test would be negative if implantation did not occur.