Is my girlfriend possibly cheating on me?

Is there a chance Facebook messenger's "device is busy" or "active now" are not that accurate because My girlfriend told me she was going to sleep last night so don't call her, later I checked messenger if showed she was active and I left it alone for awhile and checked again and she was still active. She does spend that much time on messenger so I called her through messenger to see if she was up talking to someone and it said device is busy which would mean that she's already on the phone with someone. 1 minute after I called it said she was offline. I also saw that she was active like two hours after but then I went to sleep. She never responded to me that night and when I asked her about it today she said what are you talking about I was asleep the whole time like I said. Then I asked her why would it say you were active then and why would it say your device is busy. She's denies and said that she left the Facebook tab open on her computer. But it wouldn't say that her device was busy if she wasn't on the phone right? And wouldn't she remain active on messenger until she closed the tab? If it stayed open all night then it shouldnt said that she was offline for a hour or two Hours later and then say she was active. I know may I sound clingy but this is sorta not the first time this happened and it bugs me. I really need other peoples opinion on this, what do you guys think? Could she be telling the truth? Is there a chance she's cheating?