If you have some bump in your chest, what you think its acne?

Skin lesions: Well, with the limited description acne can be on the list of things that cause bumps on the chest. I would recommend having a physician take a look to give you a better answer.

Need to examine: It is not feasible to provide a meaningful opinion without taking additional history, and physical examination . It may be acne. It could also be a lipoma, folliculitis, an inclusion cyst etc. If the "bump" doubles in size, becomes painful, ulcerates, or other bumps appear, or you develop fever, or lose weight, you should see a doctor.