Quite possibly and perhaps the cause might be RSI, or repetitive strain injury.
We cannot diagnose, so do consult a physio first.
The pain can persist for long periods of time (months) and can become extremely difficult to reverse if left untreated, so please try to adapt your hand position, or at least learn how you might be able to alter it whilst playing and apply some Atrogel® afterwards to help the inflammation.
I'd be surprised if Cellists didn't develop upper back and/or neck pain from prolonged periods in the posture.
If you have suffered such discomfort, please try some of the exercises described in the video blog on my upper back pain page.
Best wishes Maggie and I hope nothing gets in the way of your passion.