Hips (sciatica) hurt most and I find myself squirming. I also try to break up the sitting: ironing board is in the hallway, design board on the wall and cutting mat across the room. A dancer friend gave me a trick that works really well. You take 2 quart size freezer bags and put crushed ice in them and then added 3tbs. of rubbing alcohol (fill the bags only 1/2 full). Put the two bags into a gallon freezer bag, wrap a towel around it and sit on the bag for 20 minutes. If I'm really hurting I do this several times, waiting an hour inbetween. It helps the muscles and nerves not get inflamed. I hate cold so I throw a quilt in the dryer and wrap myself up in a nice warm cocoon while my butt freezes! Sometimes I cheat and sit on the ice while I sew.

I just went to the doctor today for this! He said I had glutteal(?) tendonitis on the right side. He gave me some stretching excerises that I'm supposed to do multiple times a day, holding the stretch for 3-5 minutes at a time. I actually think mine started after visiting my daughter and going up and down (multiple times!) gigantic stairs for 2 weeks. When I sew, I kind of perch on the edge of my chair and that seemed to aggravate my condition where it feels like that muscle is burning. It even affects my driving now too, since it's on the right side. Good luck with figuring out your problem so you can enjoy sewing.

Angle does make a difference. I can't sit in a booth at a restaurant because of the angle the booth forces me into. Instant pain.

I used to be a systems engineer, and sat at a computer 5+ days a week, often for 10 - 12 hours.

My tailbone was so sore, but nothing showed on x-rays or mri. No treatment available because what the heck would you be treating?

Then my company did me the supreme favor (I mean that!) of involuntary separation - medical included and a bit of a payoff for years of service.

So i didn't work for 10 months. Loved it. About 8 months into the off-time, I noticed that my tailbone didn't hurt any more. My guess is that I had a chronic bruise type injury and that getting off my duff was the cure.

I have fibro, so I'm sure that was a contributing factor. Now I notice that my hips get sore if I sit too long. Most likely osteo arthritis. So I don't sit too long any more. My pressing station is about 10' from my sewing machine, and I cut fabric at a 36 inch height craft table. The more I have to get up, the better off I am.

Best advice that i can give you is to see a dr.

If I'm going to be standing all day I change shoes, if sitting I change pillows under me or chairs. Helps a lot, so does getting up to iron!

I use a donut hole pillow in my comfy computer chair

At home, my sewing machine is sunk in a cabinet and I have an awesome Koala chair that really is the most comfortable I've found. However, I'm taking classes at the Houston Quilt Festival and the machines are up on tables . . .and the chairs are hard and YES . . .my bottom hurts . . .and my lower back. In fact, it was so bad, I returned to my room for the night at 6pm, LOL

This discussion has helped me figure out why I couldn't bring myself to do much machine sewing in my RV.
The ergonomics were terribly wrong! Machine on tray table in front of couch. Enuf said!
Luckily I had taken my GFG hand stitching project along so I still had a project I could work on. Also got lots of crocheting done.

The human back wasn't made to be in a sitting position for hours on end. I was taught to use my left foot on the peddle. This from a physio. Felt a little strange at first but I don't even think of it unless someone mentions it. Lots of breaks away from the machine, the right height table, the correct chair and sitting in the chair not perched on the edge.
I'm no expert and when I was young sat in some peculiar positions and sewed for hours even when my back was burning. Did my back no favours.

Having suffered bursitis in my left hip and now suffering from tendinitis in my right elbow, I absolutely understand the amount of pain involved. I have found that wearing good quality runners with good support made a tremendous difference. I have a pair that I use as my 'house shoes'--they have never seen the outdoors and I don't have to worry about dragging dirt around the house. It is surprising what a difference they make--to my posture, my leg fatigue, my lower back pain--even while sitting.

I bought a comfortable chair at Ikea so I never have this problem.