Hi and thanks for your question,
If you are using condoms correctly they are 99% effective against pregnancy. A negative pregnancy test 19 days after your expected period indicates that you are not likely pregnant.
Irregular menstrual cycles are very common and can be due to a hormonal imbalance, which can be affected by things like changes in nutrition, stress, activity level and certain medications.
A normal menstrual cycle is 21-35 days long (maybe 5-7 days longer in puberty). The duration of flow is normally 4-6 days long and 78% of blood loss occurs in the first 2 days of menstruation.
If your cycle continues to be irregular you should see your doctor for an evaluation.