very unusual problem...never had this happen

hi all..im 28 yrs old and have a 2yr old daughter that was born approx 3 months early due to me having severe preclampsia. I honestly had no idea that i was even pregnant, (still had my very irregular period, some months i would have it, some months i wouldnt) until i was in the ER with severe problems-resulting from the preclampsia-i was close to 5 months pregnant. 2years prior to having my daughter i also had a miscarriage around 14 weeks. After having my daughter my periods have became regular-give or take a few days. My question is, I had my period around the 27th of November-on time. However i had sex with my boyfriend on the 9th and 10th of December. Yes we had sex with a condom and also without one. I am also not on any b/c. Now the real problem, on the 12th of December, i started my period-that's 2 weeks early!! According to when i should start it, it should be round the 24th of December. Never has this ever happended after we have sex. We also dont have sex that often, maybe once every two-three months due to him living out of town. However I dont feel crampy-basically no symptoms of my period. It is very light in color, almost a pinkish orange, and it's not heavy, like they usually are. Im just clueless as to what is going on. I have looked on here for ppl with the same prob, but no avail. However, i did ask a few of my friends, and they suggested I take a hpt. I bought one lastnight the 13th-of course it came back negative. Im pretty sure it was just taken too soon. However i asked my sister what her thought would be-she has 3 kids, twin girls that were born at a lil over 25 weeks, and a son that was born 1 month early, and also had 2 miscarriages., Anyways she mentioned that when she was pregnant with her son, that she also had the same thing happen to her-early period. So is it possible that i could be, or just some freak thing that happened. Also as you can see that we tend to have problems carrying children to full term. All the women in our family has had atleast one miscarriage. Also i should mention, when i was pregnant with my daughter, i had three negative results, including one at the ER, thankfully the ER doctor demanded a blood test. Btw i dont take b/c because i get very bad side effects from it- I have tried multiple brands and types.