Hi all

My gf had a week long period and towards the end of it we had unprotected sex. 3-4 days later she had her periods again, which now lasted for three days. She is on birth control and is 19 yrs old.

She told me that she's never had two back to back periods. Also, the fact that the second period was much shorter than the first is worrying us. She did say though, that the second period felt like a regular period (no spotting, etc.) 

What are the odds of her being pregnant?