Usually the stressor is obvious, but sometimes it's not and it's something you've left unresolved that your subconscious gets caught up on from time to time.
It might not be the talks themselves. It could be anything.
If you've never had a panic attack before and start getting these, it literally feels like you're going to die the first time you get one. Then you get used to them and start adjusting.
I went through a period of panic attacks after acquisition talks with a giant corp fell through. I was messed up for years after that, with random panic attacks at all times of day.
I worked on returning myself to my previously chill demeanor and haven't had one for many years now.
I also stopped answering any VC/BizDev/CorpDev emails though. lol. But mostly out of a desire to optimize my time use for things that actually matter.
Good luck Martin. Covid has certainly put the value of living the life you truly want for yourself into perspective and I hope you achieve that.