Shortened Period

My girl friend and I have been dating for just over 5 months now and I took her virginity just before our one month. Since then we have been having sex on average of about 5 times a week using condoms yet the longer and longer it goes the shorter and shorter her period has been getting for the last 3 months. She used to go the full week and it is getting the point where she is only going for 2-4 days and hardly anything is coming out. Should we be worried? She is freaking out to the point where she wants to stop having sex completely and I donno what to tell her. She is only 17 and still afraid to say anything to her mom. What should we do? Please help.