Without seeing the actual bruise it is hard to determine if it is really a bruise or post-inflamatory hyperpigmentation. Normally, we do not see bruises three weeks after a Fraxel, however there may be that effect from other treatments such as Ulthera. Are you on blood thinners? Then I would understand this result.
If it is simply a change in pigment, which is normal, you can be treated with the Vbeam for hyperpigmentation; however it should even-out naturally within 4-6 weeks.
When you go for a factional laser treatment, your doctor or technician should let you know what you can expect in terms both the treatment and the post-treatment downtime and results. You should go back to your clinic and inquire what is really going on.