8 mnth old woke up screaming in pain not able to move leg due to pain. diagnosed with ear infection, ear infection cleared with abx but leg pain continued then diagnosed with osteomyelitis. how does osteomyelitis come on so fast? he wasn't sick prior?

Osteomyelitis occurs after germs spread through the blood & end up at the bone/joint. Because the ear infection is common & bone infection rare, it may have occurred at the same time and been overlooked, or just not evident on exam. There is always the day before a patient develops their appendicitis, when they have no symptoms, the same here. The same germ can cause both.