Try to get filmed to see if you are bending your back too much.
As others said, you need to bend your knees, or just learn to paddle with the upper hand lower...
Better paddle with your lower back straight but the hand under your chin:

than bend your lower back and have you hand above your head:

Note that keeping you upper hand low means you will have less reach (the lower low hand will limit it)... just learn to use smaller strokes, your lower back will thank you!