Earlier than expected?!

Went to the doctor today, 7 weeks 2 days, and saw a fetal pole for the first time... They were unable to see it when I went at 6 weeks 1 day. Today when they measures, they measured me at 5 1/2 weeks... I'm so confused. The one doctor at the perinatal office said its nothing to be concerned about because there's been development. But my OB told me she's concerned of a 50/50 chance miscarriage because I have fairly regular periods. I'm so confused and just need help. Has a situation like this ever happened to anyone else???