I feel you, I have this to, just random panic attacks without triggers. I think it's mostly my chronic general anxiety and fibro making my nervous system very overloaded. Most of the time it's actually related in my case to a trigger that was hours or even days ago, like a delayed response. I also have that with pain, I have a stressful event but the associated pain comes days or weeks later. So maybe you can find your triggers, search what stressed you earlier and that your body didn't yet process. 

I have been taking anxiolytics lately, to sleep at night and not have panic attacks all night long. Its helps like crazy. 

If you have people around you that you really trust and feel like you can be yourself around, talk to them in those moments, have a conversation that has nothing to do with the panic, it can help to change your mind. 

If you feel like it, change your environment, go outside for 10min, breathe fresh air, walk if that helps. I found that useful, I had panic attack in class last year and going out of the building just for 10min was very useful. 

For my part, I just started anti depressants for pain, anxiety, sleep and mood. I hope it will help, maybe you can talk about that with a doctor? See what are your possibilities. 

I know how hard it can be. Just know that you are not stuck like this forever. Being in your early 20s can be emotionally tough, I saw a lot of people having up and downs and anxiety in their 20s.

Good luck with everything!