Help and reasons for major stiffness in lower back when getting off my mountain bike??

Hello

Basically, for about the last 6 months when ever I lean over or if I round my LOWER back whilst doing anything such as Bike riding it becomes very stiff with a dull ache and takes a while to straighten up. (If Im sat at the table and Im up straight and then let my pelvis rotate back so the very very lower base of my spine rounds then this is another scenario when it stiffens and takes longer than normal to then straighten back up) But bike riding is bad when I get off the bike.

Any reasons why this is happening and what the possible problems could be.

Thanks
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