If you're not triggering it with feeling anxious, I'd say its pretty normal and not anxiety. I had a really hard time with not being able to breathe around the 2nd/3rd trimester mark for several weeks. It felt like tightenings, like I couldn't fill my lungs completely. I hated it, absolutely hated it- would happen with very little exertion. Bad bad reflux around the same time.

I would gently stretch my arms over my head and breathe deeply. Also a little bit of cat-cow yoga movements as it stretches the spine and made breathing a little easier. Try a little squatting on your flat feet, again while stretching your spine, as that seems to create space too. The other one I did a lot was I'd lie on the ground with my feet at a 45 degree angle on the lounge or whatever, with my arms stretched out on the floor above my head. Use a pillow/pillows under your head and back.

The good part is there's hope, your baby will drop and you should be able to breathe again.