Increase in physical energy on third day of Prozac (fluoxetine) 20 mg. Is this a good sign? I don't feel like it's hypomania or mania.

Yes. It is a good sign, although early to be an effect there are some who respond sooner than others. Is there someone close to you who can observe and let you know if you are becoming hypomanic? Symptoms to be concerned about are irritability, sleeplessness, overspending, sexual promiscuity, angry outbursts and very poor judgment. I doubt you will experience these. Happiness, however, is desired.