symptoms increase --after the FCE -- make sure your pain and symptoms get documented and hopefully observed by a PT or MD while it is still happening.
The infamous Delayed pain of TOS!

What you describe sounds like spasms- does your neck feel kind of "locked up"
like to turn your head it very difficult or possibly can't even do it?

A good Dr or PT should be able to feel/recognize spasms, and recommend the best treatment.

What my chiro did was a mix of trigger point and tissue release on my scalenes and SCM, it took a couple of weeks 2x a week to get rid of the most discomfort.
But I didn't know what it was at first and just suffered with it until I happened to meet the chiro at a booth at county fair one of my lucky days
He also did some release on my pec major below the clavicle, used the IF stim on my whole back.

But what happened to me was the SCM & scalenes spasmed so much that they pulled my top rib up and it got stuck like that and until 1 adv PT guy did a rib mobilization and my top ribs dropped a half inch back down to normal
gosh that was instant relief from many lingering sx.
That was my big turnaround
now chiro checks my top ribs every session.

but my main point is that these spasms can be treated by smart and caring PT/DC/ DO or even a good massage therapist.