Help! All of a sudden I can't Breath

I've been a noob having a wonderful time (started in early July). I've been very happy progressing through drills to finding my cadence and up to swimming for 60-90 min. I went through a period when I was struggling to get my balance and breathing right at the 10-20 min mark, but that seemed to be behind me. Love losing myself in the rhythm of it all.

Over the last two weeks I've been on a downward trajectory where my chest just tightens up and I can't seem to swim through it. Tonight I did barely 5 minutes before I froze up. Spent the rest of my time doing drills.

Interestingly this correlates with a big increase my fitness levels in my other sports. I am on a long term assignment in the Bay Area with access to some fabulous hills and beautiful country for running and riding. I've always loved climbs, but was trapped in BORING NYC for the last few years. I've been having a great time, but my swimming is suddenly a horrible struggle. I am wondering if my breathing patterns in my other sports are screwing this up.

What I've tried:
* Nose only exhale (my usual once I am in rhythm on a long swim)
* Mouth only exhale
* Mixed exhale
* Spending more time in sweet spot, but I still seem to get dizzy with that
* Newest tactic... completely effortless swimming. Trying to do a stroke so mellow that it has no impact on my breathing. So that I can just focus on the body motion of it all. Just started this tonight so not sure if it will help. It is certainly a worthwhile drill for technique more generally.

All advice welcome.