My first question is what do you do for CORE exercises?? The Core is the STAPLE of any lifting, people become to dependent on belts and suits. Personally, I NEVER wear a belt unless Im doing a heavy 1 rep Max single.
My first step would be evaluating your core work brother.
If you had an MRI of your back and it showed no structural damage this is most likely a soft tissue issue. Spasms when bad can be COMPLETELY debilitating and almost worse then something more serious.
Find a nice deep tissue massage therapist. Hot Stones would be a help as well for the target spot your having problems with.
Since your MRI was clean I would find a good chiropractor. Any Chiro worth a piss will require a set of XRays and they can give another perspective.
Stretch, Ice, Heat, Hot Tub, Sauna.
If it hurts dont do it but from you post I get there is no Disc problems, or anything else that I would deem serious.
My next question for you is who did your Back MRI or CT? Was it a general Orthopedist or a Spine Specialist??? If it was a general orthopedist I might ask them to send it on to a Spine Specialist.
If you have any other questions Im not an MD but have worked in Ortho Clinics and Surgery for 8 years with Rehab and Therapists so I can maybe help in pointing you in to the right direction.
Keep lifting man.