Should I question her about cheating? I don't want to be with her if she's with other guys.
I don't think that is a good idea. If she is, she will deny, if she isn't you will look weak. It's a no win IMO.
But if you think she's cheating, then I would listen to that gut.
Where are you at with the last contact with her? When you do talk or spend time with her, you should be calm and cool.
Keep in mind all the advice you get here, it's guys who are coming from various experiences they've had, and they want you to be in the best place for yourself. You will want to flow from this place of confidence and a position of control when you are with her. That doesn't happen on the spot, so make sure you are taking care of yourself with exercise, eating well, and keeping BUSY during this time where she's pulling back (it is challenging to be in this place with a woman you care about).
Be prepared for her to say she needs "time" or other breakup type talk. *If* that happens, you will need to walk and not contact her for awhile. And in that moment you need to just be calm like it doesn't bug you at all.