Hi
Welcome to the forum!
Sharp stomach pains in the epigastrium (upper abdomen, if this is where you have pain) in the presence of blackouts and weakness should raise the suspicion of heart problems. This needs to be ruled out by a 12 lead EKG ASAP.
Acidity and GERD too can cause blackouts which can give a feeling of numbness in the upper body.
Another possibility is thoracic outlet syndrome, which happens due to compression of nerves or blood vessels, or both between the base of the neck and the armpit (thoracic outlet).
Discuss these possibilities with your doctor. It is difficult to comment beyond this without examining. A comprehensive investigation is required keeping all the points in mind.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
Sharp stomach pains can be as simple as gas, or much more serious. Have you had your CBC, iron and ferritin blood levels measured lately? Have you ever had a brain MRI? Have you ever been tested for blood sugar levels? Is the lightheadedness associated with standing up?