What are bumps behind the nipple of a 11 year old male?

Males and females make both female and male hormones. The ratio we make depends on our sex. Males make more testosterone than females and females make more estrogen than males.

When puberty starts, production of both increases with the dominant one really increasing. Because of this, when a boy starts puberty he also increases his estrogen production. In many boys (about 20%) this causes a temporary growth of breast tissue, a condition called gynecomastia. Nipples will swell, they become more sensitive, they might even get actual breast growth. Fortunately, it's temporary and usually doesn't last more than a year. When testosterone production is at its peak its ratio to estrogen is much higher and the breast symptoms go away.

It's the same reason why older women sometimes begin to grow facial hair (estrogen production drops and the ratio is closer), and men have less facial and body hair (testosterone production drops and the ratio is closer).