My period is 20 days late?

There are several different possible explanations for why your period may have been late. If this continues to be a problem,

then it would be a great idea to see your primary care doctor or your OB GYN doctor for additional workup. One possibility is that your period may have been late because of stress

fatigue, an acute illness, or a change in your sleep patterns, and all of these can affect the hormone signals from the brain to the ovaries that regulate the period.

Another possibility is that you may have been pregnant, but that you subsequently had a miscarriage of the pregnancy. Very early miscarriages are extremely common, and they may have no overt symptoms (like heavy bleeding or cramping) other than causing the period to be delayed.

A very early pregnancy may be too small to cause the pregnancy test to be positive. There are numerous other hormonal causes of delayed periods, such as problems with the thyroid gland or polycystic ovarian syndrome;

however, these don't typically need to be looked for unless the irregular periods are occurring frequently, at which point you should see your doctor. It's a good idea to book an appointment your physician!

You don't make sense. "usually always at least a week late"

Is it usually? Or always? If you're always late it isn't late. See what I mean? You're not regular anyway.

Your mom said you don't need to see a gyno? Your mom's a doctor?

At 17 you're old enough to potentially have problems. See a gyno if you're worried.