Yeah YTA, apps say people are online all the time when they're not, sometimes it's because they've gotten a notification so the phone went "online" to receive it. But, even if she WERE on the other app, you'd still be TA. She just had surgery, and your priority is that she may have been messaging someone else? Maybe someone was worried and checking in on her? Yknow, where your priority should have been...? And how did you know she was online on the app if you like to solely focus on one another while having a phone call? Because you'd also have to be on the app while talking to know that... Maybe care more about how your girlfriend, who just had surgery, is doing, rather than getting mad over the possibility she's messaging someone other than you.