Delta V
That is strange as I had a similar experience flying with a PPL who was IMC rated and a heavy smoker to Ireland.
The cloud base was 800 feet and I suggested we filed IFR and flew at FL100 in his SEP.
In Solid cloud passing 5000 feet he became uncomfortable and his breathing rate fast.
At FL100 we were in the clear blue above a solid deck of cloud. He kept asking if we could descend to 5000 feet even though that would put us in cloud and lower over the sea.
I told him it was crazy to do so and reaching the IOM the cloud broke up revealing the sea below. seeing the sea he was happy again.
On the return he flew VFR below the then 3000 foot base and was happy as Larry
Pace