i also had the same problem when i started bench pressing. IMO the force of the weight u lifted was transfered to the lumbar vertebrae aka ur lower back bones.

what i did was try to rest my shoulder blades on the bench, and during pushing the bar up, i looked down at my chest trying to observe its contraction. try it out. it worked for me

take care mate. drink lots of water and pee like there is no tomorrow. don't worry about workout. take it as a deconditioning period