oh ok. I have spondy so my spine isnt straight. I used to have the same problem laying down and relaxing my back. the relief was so good but equally intense pain lol. for my back, it was because i was tense all day trying to move and hold myself in specific ways to not squish my spine further. relaxing from that squished my spine. so depending on the type of pain, you could be similar? like your body/muscles are used to behaving a certain way so relaxing them feels wrong. kinda like when you stretch something you dont usually stretch. but more ouchies. Or your feeling like cramping from clenching so hard all day. maybe setting a reminder/timer of sorts to remind you to relax/stretch throughout the day? a little bit at a time might help tone it down