Lower Back Pain After Physio

Hi, I'm 21 years old, 5'10" and roughy 80kg.

I've been getting a sore/tight lower back for a while, it happens when I've been lying down, however it also hurts if I try bending at the hips (best way I can explain it is with either a deadlift or squat form.)
I had physiotherapy and was told to work on my posture, core and to carry out an exercise (lie flat down and push my upper body up with my hands/arms but keep my hips on the floor - I was told to do this until the sharp pain in the middle of my lower back disappeared)
Having followed all of this, my posture is better, core strength is good and I no longer feel a sharp pain when performing the exercise, but the tightness is still there. The best way I can explain it now is if I lie on my back and start to bring my knees up to my chest, as soon as I put pressure on my knees with my hands to bring them closer my lower back feels very very tight.

Im not sure what to do now, any advice would be greatly appreciated :)