Was my ex cheating on me?

So about a month ago my now exfiancee Jennifer & I were going to be moveing in with Jeromie that I had considered a friend. So the house was still being fixed up so I decided that I was going to stay at her mom's house until things got fixed that Jeromie had said would be fixed by the time we moved in. So about three days go by & Jennifer calls her mom's house & tells me she dosnt love me anymore & no longer then two days later does she start dateing Jeromie. What I would like to know is am I right to think she had been cheating on me & been planing this for awhile.