Thanks for the nice and crisp description of your symptoms. Your symptoms strongly advocate the possibility of Ankylosing Spondylitis
which is the disease your rheumatologist
suspected, and rightly so. But a normal MRI
rules out that possibilty, with support from the normalcy of blood tests, which means, reaching a diagnosis would be difficult. But apart from that, its a good news as it means the absence of something grave. It means that you might be just having a prolonged spasm of the muscles around lower back (despite your intuition). You might need some muscle relaxant
for few weeks in addition to anti-inflammatory drugs.
It would have been better had you mentioned the blood tests too. Wish you a speedy recovery.