Hello dear
X-rays only detect injuries to your bones and have got their own limitations.
However if your x-rays are fine and you are only getting an off and on stiffness of spine, it doesn't look serious but still you need to answer a few more questions:
1. Is the stiffness in the morning and lasts more than 30 minutes?
2. Do you get heel pains?
3. Does the pain awaken you at night sometimes?
If the answer to the questions above is no then it is probably because you would have hurt your ligaments and muscles at the time of injury which will recover slowly. Just keep doing back strengthening exercises and take proper care of your back.
Next Steps
Give it 2 weeks from injury and if doesn't seem to improve, get back to me. We will figure it out
Health Tips
Avoid forward bending and lifting heavy weights