Thank you for consulting with Doctor spring. I understand that you had unprotected intercourse on two occasions after which you suspect that you might be pregnant.
Let me explain to you why there is a 90% chance that you might be pregnant. First, your last menstrual period was on 24th of November and you expected your next period on December 21st, which means that the length of your cycle is 28 days. Considering that your cycles are regular, normally you would be ovulating on the 8th of December. This is because the ovum is released on the 14th day of a 28 day cycle. In women with a longer cycle, the time of ovulation is length of the period minus 14 days. For example if your cycle were 40 days long, you would be ovulating on the 26th day.
Considering this data and the average life span of the sperm and the ovum, the safe period for any cycle is calculated. This is taken to be form the first day of the period to the 10th day of the cycle and then from the 18th day to the 28th day. Alternatively, the risk period can be calculated as being 3 days before ovulation an 3 days after ovulation. If your periods are irregular, the safe period can be calculated as
shortest cycle minus 18 to longest cycle minus 10 days gives the first and the last day of the fertile period.
Since you had intercourse on the 13th and 15th of December and your condom tore on both occasions, it is likely that you were fertilized at about this time. Even one positive pregnancy test means that you are pregnant. So when you performed your home pregnancy tests on the 22nd of December which is the time of the missed period, it indicated that you are indeed pregnant.
Since you do not want this pregnancy, I suggest that you meet your Gynaecologist and discuss the options for termination. Hope this information was of use to you.