We probably need to define some terms here. The title of "Buddha" is technically only available to one person per dharma cycle per world. A dharma cycle begins when someone awakens to the truth of existence (the answer to the probelm of dukkha) and begins teaching this awakening to others. This person is qualified to receive the title of Buddha. The dharma cycle thus begun lasts until this Buddha's teachings have completely gone from the world. Estimates of this time cycle range from 5,000-10,000 years, to much longer. When those teachings have completely been forgotten, the time is then ripe for another teacher to awaken and begin the cycle anew.
The Sutras mention many Buddhas. Depending on which Sutras you read and study, there are different explanations for this. The Lotus Sutra (with which I am most familiar) notes that these Buddhas are from other worlds throughout the universe. Buddhas often seem to (at least metaphorically) travel and visit to support each other and attest to each others' veracity. All of these Buddhas, though, have met the test of the one who awakens and begins teaching on his own, in his particular world.
Buddhists, and especially non-Buddhist folk, tend to use the word "buddha" as a generic term for anyone who has achieved the awakening that Buddhism teaches. This isn't wrong per se, but it gets confusing when mixed with the strict definition of the title (above).
Finally to answer the OP's question, Sakyamuni (Siddhartha Gautama) was the Buddha once he awakened and turned the Wheel of the Dharma (began teaching) for the rest of his life, whether he was eating, sleeping, eliminating, teaching, or meditating.