I have to respectfully disagree with the previous poster. I believe good posture has little to do with strength, not even the often-mentioned abdominal muscles. On the contrary. Strength is most often simply used to mask bad posture, to compensate for it, not to correct it. But that's really a different topic.
In any case, computers and piano mix perfectly well - as long as you carry out movements that are anatomically correct. You have a choice. You can use bad movements or good ones, and you can use them, or not, in practically any situation. Granted, certain environments help if one doesn't have the discipline. For example, you could try using a trackball instead of a mouse, getting a different chair, getting a better monitor, getting larger glasses, etc., but the principle remains: it's your choice.
How certain are you that you have good posture? Do you know what good posture is and how it feels? This is not meant as an offense, I only want to make sure that you have the correct goals in mind

If you do have good posture, do you also have good movements?
In any case, pain means there is something wrong in how you use your body. It doesn't have to be the piano. It can be your computer work that is aggravetd by playing the piano, or the other way around, or it can be something completely different. The only way to find out is to make changes and see how they affect the symptoms. To help you along, you'd have to explain exactly what it is that gives you problems and what those problems are. Oh yes, one more thing: if you are in pain, stop doing what causes the pain until it is gone, then retrain.