Why do my legs hurt after I've used my computer? It's very uncomfortable and painful. I have a laptop.

Likely positional. Most likely you are having your legs in a position they "do not like" while working on your computer.Could be they are extended and the knees locked, or flexed a bit too much. Regardless, it is best not to stay in any one position too long for many reasons.Change position, get up, stretch, walk around.But severe pain, or other symptoms can mean something that needs attention. Get history & exam.
? be you spine. Sitting places the greatest stresses on the human spine. If you are developing radicular symptoms in your lower extremities than it can be coming from your spine. See your orthopaedic spine specialist for an evaluation and an mri.