Started at 20mg paxil and I have terrible stomachaches....should I go down to 10mg?

My doc prescribed 10mg of paxil to take. However, thinking that dose is to small to have an effect I tried 20 mg of the prescription. Now my stomach is killing me. Ive been taking it aat 20mg for 9days now. Do you think if I lower the dose I will have less negative sideeffects and maybe a good sideeffect or two? lol Thanks