Is collagen found in the human body? I know that it is mammals, and since we are mammals I thought it was.?

Yes collagen is in the human body, we are animals and mammals. Just being human doesn't make us an exception to the rule. Collagen is a protein used for structural support in our muscle, skin, and other soft tissues. It is also found in our bones and cartilage.

When people get collagen injections, it is to puff up a part of the body, like the lips. However that collagen is not maintained by the body, so it eventually breaks down and the puffiness shrinks

Yes, but rarely. More common is to meet ones that will agree we are mammals, but then try to point out that we have attributes like speech, music, reasoning, etc. and are therefore special and separate from other mammals. However, they usually completely disregard the fact that each mammal species has its own unique attributes that separates it from every other mammal species. I think some people just have a psychological problem with us being a species of animal, indicating some particular need to feel special, while the rest of us are quite content with that.