Hi thanks for your question.
Causes for this delay of periods could be No 1. Irregular period. Once menstrual cycle returns after lactational amenorrhea; to begin with it can be irregular. Second reason could be pregnancy
. You are overdue only by 13 days, and in 13 day you have taken 4 pregnancy tests
. You have taken pregnancy test little too early. Pregnancy test turns out positive 10 to 12 days after missed periods. Repeat the pregnancy test, if it is still negative, pregnancy is ruled out as a cause of delayed period. If your menstrual cycle doesn't return by another 2 weeks time consult a gynecologist who may evaluate you for delayed periods.
Hope this answers your question