Was/Is my girlfriend cheating?

Backstory: I've been dating this girl since December. From about March things became more serious and we became a couple. We've both told one another we love each other and already we've discussed things likes marriage, kids, all that sort of thing. Things moved pretty quick between us as you can see. We both want the same things in life, and we both have an incredible connection with one another. I've never felt this way about a girl before.

Last night when I was in hers cooking dinner together a notification went off on her phone. I recognised the tone. It sounded like a Tinder notification. I had been on Tinder last year so this is how I knew. I even checked Youtube just to play the Tinder notification and yes it was the same thing. I didn't confront her initially as I was still unsure. Maybe I misheard it. I checked her phone about an hour after the notification just to see if there was something there. I've never checked her phone for anything previous as I've had no reason too, nor have I been that type of person until last night. There was nothing there. This morning when she was in the shower I checked it again and for some reason she had put her phone in airplane mode (no wifi/no mobile data).

When I got home I asked my friend who has Tinder to narrow down a search to see if he could find her on it. He found her plus 3 pictures, one of which I recognised as she had taken the picture to send to me at the very end of January on a night out. I rang her and confronted her about it. I went through step by step everything I've said above and she admitted to it. She was so calm. She said she hadn't used Tinder in months and was free to do what she wanted to do whilst we were only dating as we weren't an item then, even though back then I was upfront with her and told her I really liked her and we shared some special moments together. She said she's never had sex with anyone else nor been on a date with anyone since dating me. She passed off the notification as being an old one, which is impossible as I know how Tinder works (it was either a message from a guy or a match). She said the reason her phone was on airplane mode in the morning was because it charges quicker. She also only got a new phone a few months back and said her phone automatically downloaded and logged her in, even though the latter is impossible as she'd have to accept all the requirements on a new phone the first time of logging in. She remained calm throughout like she had done nothing wrong and said she just had forgotten to delete Tinder not was it a big deal she was on it still. She told me I was overreacting and she had done nothing wrong. She said when I calm down I'll realise that.

After that I don't know whether I am overreacting or was there grounds for cheating. Please, please help.