My back was in so much pain this month. Doctor said "myalgia" which means muscle, but I think it could be joint pain. Xray was clear, showed no problems. What does your Crohn's-related back pain feel like???
X-Ray will only show joint damage which is a result of long and ongoing aggressive joint inflammation (arthritis). Arthritis can not be diagnosed with X-Ray. MRI would be an option, best option is ultrasound for smaller joints and knees etc. (MRI for back).
When joints are inflamed, tendisitis is very likely as well, so all the tissue around, including muscle can get very tight and trigger points are sensitive.
Severity would be dependent on joint inflammation for most of the patients.
When my joints are super inflamed - like now - my whole body aches kind of, I feel like being super heavy, legs feel heavy, muscles feel weak and heavy (I can still swim and exercise on a lighter level but I just feel like being 120 years old, as if my body isn't strong enough for my weight, which makes no sense because I'm slightly underweight...).
Would recommend to see a rheumatologist and ask for ultrasound and / or MRI.