Thanks for the query.
As your cycles are irregular it is difficult to guess the period of ovulation. Anyhow as you had previous cycle with 21 days duration there is less possibility for ovulation on 15th Dec
As you interfered with natural cycle by administering hormones from outside in the form of contraceptive
pills, now you may not get your menstrual flow as expected before.
So you wait for two more weeks and go for blood test
or go for urine test for pregnancy after 15th Jan.