We are in relationship for more than two years. She went back to her home during college vacation. While there she planned to go and meet her ex-boyfriend, who is also a school friend, to remember the old days. She went to his house to meet him, and they had sex together. Later she also told me all these things. She told me that her ex-boyfriend forced her to have sex and that he wanted her back in his life, but she didn't want to go with him. She wants to continue the relationship with me. What should I do? Please help.
I assume that you and she have been having sex since you say that you are in a "relationship." If that assumption is wrong, please forgive me drawing that conclusion, but I have been finding people vague about what constitutes a relationship.
The core of your question is whether you trust your girlfriend. You know that she claimed that the sex with her old boyfriend was forced, but you aren't sure.
The problem is that, while you would like to think of your relationship with her as exclusive, it isn't. There is no commitment or covenant that obligates the two of you to remain faithful to each other. You and she already violate God's teachings by having sex without being married. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Thus, if she also sins with another boy, there isn't much difference.
I can't tell you whether the sex with her former boyfriend was consensual or not. I could see a boy thinking he could prove his love to a girl by a physical action -- having sex with her. That sex isn't love is something many people have a hard time grasping.
If you want a good relationship, then you need to start putting God's laws into practice in your life. That means the sex has to stop. Then you will be able to focus on getting to know each other as people. It will also give you an opportunity to decide if you can be loyal to each other or whether sex is more important.