Given the data you provided, your getting pregnant might be unsuccessful due to the irregularity of the length of your menstrual cycles (from December, 21st until January, 23rd your cycle lasted for 33 days, and from January, 23rd until February, 14th the cycle was 22 days long-from the first day of your period until the last day just before the beginning of the next period). Ovulation is every time the same: you calculate the exact ovulatory day from the day of the expected onset of your next period minus 14 days, and to this calculated day-plus/minus 2-3 days. But, unfortunately, this might be more usable when a woman has regular (in length) cycles and knows which day her next period will come. In irregular cycles, you can not predict which day exactly will your period come, so that you can from that day subtract 14 days and have your ovulatory day(s). It might be recommended that you visit your gynecologist around the presumed ovulatory days. The gynecologist could do blood hormonal tests (since you have an acne problem, it might be possible that there is a sexual hormonal imbalance behind that, so s/he might want to start you on hormonal medications) as well as measure the Graafian follicle (the biggest egg in the ovaries, waiting to be expelled) and by its diameter, s/he could tell when exactly the ovulation will happen. Also, to mention: in case you take any anti-acne medication, certain of these medications could be harmful to the baby and should be stopped before getting pregnant. You might want to consult with your dermatologist/gynecologist about this issue, too.