Consult must . Stretching exercises and hot fermentation for you

Do back muscle strengthening exercises. Drink plenty of water to avoid dehydration because dehydrated disc is more prone to injury

Primary reason is your flexibility and poor mobility in spine. Disc between the two bones in spine has a special hydrophillic {water absorbing} property when it is not loaded to much. During night spine stays relaxed so disc absorbs more water,  as the disc{swollen Ballon} are stacked between two bones of spine ,beacuse of increased water content spine is pushed to bit elongated state, which your joints in spine ,muscles surrounding spine , covering of muscles (Fascia) are not allowing because of poor flexibility and mobility. That is why you feel pain in back . Within few minutes of body  load and movement water moves out you feel relaxed

Postural Correction- Sit Tall, Walk Tall. Extension Exercises x 15 times x twice daily. Apply Hot Fomentation twice daily. Avoid bending in front.

It may b muscle imbalance impact on spine

Morning low back pain should be evaluated for other problems also

Hi, if it's not throughout the day or any other activities, definitely your mattress may be the cause. You should try using a medium firm mattress as your spine has to rest in straight position.. or can sleep on a flat surface with pillow underneath your knees.

Pain in back at early morning may be due to improper sleeping position check it out and see the next day how it work out