I (14m) have PTSD and therefore get very anxious/scared about specific things such crowds, public transport, lifts, etc. 

However, recently (I had this about a year ago, it got better, and now it’s back and much worse) I’ve been randomly feeling anxious when there’s no trigger. 

It’s not exactly a emotion; there’s no cognitive component and I can feel actual emotions on top of it normally. More, it’s the physical sensation of fast heart rate (resting is 97, here it’s 110-115), tightening chest, sensation of adrenaline in muscles, etc. It’s generally almost ignorable, in the same way a headache is, but sometimes it does lead to SH. 

What is causing this? Is it the beginning of mild panic disorder?