If she is distant, withdrawn, constantly cancelling plans, and appears not interested in what you have to say or do, then chances are something is up. Observation is the key to finding out what is going on. This does not mean you go paranoid and let your suspicions get out of hand. Don’t read too much into things and take her behaviour for what it actually is
Another alternative is to talk to her close friends; provided you are on talking terms with him/her. Don’t pounce on the friend; call them and ask them if they genuinely know what is happening and if they have any idea what is bothering your girlfriend. Try to coax information out of them and see where that leads
How many cheating men and women have been caught thanks to phone records that gave up on them. When you get a chance, look through her phone and you might just get the proof.
This is the sure shot way to find out what is going on. Confront her, coax her to talk about it and get information out of her. You will know the 100% truth only when she talks. So you got to ensure she does. Use these ways to know if your girlfriend is actually cheating on you or if it’s all in your head.