toddler back pain swollen lymph nodes can't sit down

My 20 month old daughter has had lower back pain for the past three weeks. No fever. She stands all or lies down all the time because she cannot sit without pain. She can move her legs and flex her hips, but if she sits or if I lift her bottom up while changing her diaper, she whimpers in pain. She also will not bend forward to pick up something in the floor. She holds her back straight and squats or holds onto something and gets on her knees or belly to pick objects up. She cannot get up from a lying down position. She has fallen several times face down and cannot get up without help. The original diagnosis from her pediatrician was toxic synovitis, but this was ruled out after they realized the pain was in her back, not hips and after it lasted this long. She also had some abdominal tenderness the first week, but this disappeared. The pain wakes her at night and she is getting only 3-4 hours of sleep per night/day. She cannot get comfortable even in my arms for more than a few minutes. She was in the hospital for three days for testing. Had fluids for dehydration. Has had MRI of head, thoracic and lumbar spine, CT scan of abdomen, x-ray of pelvis, spinal tap, and bone marrow biopsy. All tests were normal except for very minimal inflammation around the bottom of the spinal cord that wasn't even noticed on first reading of the MRI as well as swollen lymph nodes in chest, abdomen, and neck. All cancer tests have come back negative. Negative for mono, CMV, and meningitis. waiting on viral culture from spinal fluid. Some white blood cells found in spinal fluid, but the puncture was "traumatic" so they're unsure whether this is a result of the puncture itself. Was tested for UTI during the first week of illness and was negative, had another test yesterday which was positive for e.coli but doc thinks this is unrelated since she was negative earlier in the episode. Had a TB test with a 7-8mm induration on arm after 48 hours. Two different docs say this is not a positive result. Referred to pediatric rheumatologist in about 2 weeks if no change in symptoms.

Blood tests show mild anemia, moderately elevated leukocytes, slightly elevated sedimentation rate. No other significant findings.

Pain did not respond to tylenol and was moderately reduced by ibuprofen. Current naproxen perscription has provided the most relief and she is now able to sleep. No change in symptoms when unmedicated.

Any thoughts or suggestions?