I realised that I was just relaxing on my phone but I got aroused due to nothing but the thing is I don’t know if I wanted to or not. When I realised I was I tried to stop and it did. Is it a sin?
Anything involuntary is not a sin, especially since you immediately halted yourself once you realized you were getting aroused. Don’t worry about your body’s natural habits; just don’t voluntarily encourage lust or do any deliberate sexual sin (masturbation, porn, extramarital sex).
Ahh well I certainly didn’t do anything like that but if I did want to get aroused I didn’t know or I just didn’t have a clue.
If you didn’t even know you were getting aroused and just “realized” it, you’re fine. Be at peace :).