I think my gf is prego?

okay so me and my gf broke up 2 months ago and we havened had sex for a wile she told me that she has had 3-4 periods and they have all lasted 6 days normily. i think the last time we had sex was super bowl sunday. she doesnt look at all to have gained wait and her sorry if this sounds bad but her boobs still look the same. she told me she was sick when she woke up this morning and didnt go to school. im thinking this is in her head or shes trying to scare me but im still worried bc im only 15 she is 16. also we used a condum every time and i alwaysed checked to make sure it didnt break and it never did it was always filled like it useually would be. also she has been trying to get back together with me is she just trying to get me back? she told me she was going to the hb to finde out for sure. her mom said it was inpusible for her to be prego affter she told her mom the story. she hasnt missed a period i know for sure the first 2 bc i was still with her