When i sit on chair at computer for just 30 min., i get lightheaded/weak/shaky. feel better after getting up. what could cause this? what type of doc to see? i also have l4/l5 disc issues- not sure if related

Light headed : I have several ideas. Severe Pain from the back can cause this. This sounds like orthostatic hypotension, blood pressure that drops when standing, cause dehydration, autonomic nervous system problem, circulation issues. A side effect of blood pressure or other medications.

Lightheadedness,shak: It would be very helpful to know what your blood pressure, heart rate and blood sugar are during those symptoms. If it was orthostatic hypotension your symptoms would most likely worsen on standing up(due to bp dropping) instead of improving. Start by seeing a primary care physician who will try to reproduce that scenario & take those measurements including sitting & standing bp, blood sugar,pulse

Couple of leads : Your symptoms suggest an early migraine like attack. 2 things I would recommend would be checking your vision at an optometrist office, followed by use of protective screens as recommended, second would be to raise your computer to eye level while standing. That should help with deciding if your position is involved, sitting/standing.