I get the exact same thing. It freaks me out which makes my anxiety worse and then I really feel like I can't breathe. I've found ever since I started getting anxiety and panic that my breathing pattern is really screwed up and that I tense up my stomach muscles and do not take in enough air. For exammple, ever since I started the panic i've been afraid to exercise cuz i'm afraid my heart is gona beat to fast its gona conk out. So even at school when I have to climb the 4 flights of stairs twice a week I get scared and I start to panic and then I feel like I can't breathe. But one time I was distracted cuz I was thinking about sumthin else and I was breathing normally and I climbed the stairs no problem and I was a little bit tired when I got up there cuz i'm horribly out of shape but I could still breathe fine and I wasn'tt panicked. Anxiety can really f**k u up!