You had two periods in one month since your body naturally starts a period after the hormones stop being provided. The breast tenderness and cramping could be from the pill or from taking the pill improperly.
So long as you've been using the pill properly (taking all active pills at the same time daily--cell phone alarm helps with this--and not stopping in the middle) for a month and are not on your placebo pills, you SHOULD be safe (note that should is not a guarantee as there have been people who get pregnant on the pill), but this is only if you are using the pill you have properly. If you don't have a boyfriend, however, and Valentine's Day is going to be a random hook-up, please do use a condom to protect yourself further. It's not just pregnancy you'd have to worry about then. If you know your partner and are 100% certain they have no STDs, then the choice is yours, but honestly, it is a risk.
The bleeding that you had on 28th dec and 11th jan was due to breakthrough bleeding due to hormonal imbalance caused by stopping the pills and not your period bleeding. The symptom that you are having are due to these hormonal changes. However to confirm the status of pregnancy, I suggest you to test for pregnancy by a highly sensitive pregnancy test 10 days post ovulation with a sensitive ( 20 - 25 mIU ) pregnancy test. Also then I would suggest you to get your serum HCG levels estimated since this is more specific than urine for pregnancy tests.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.