"I feel fine."
Translation:Ego bene me habeo.
"Ago" is the first-person form of the verb that means (among other things, many verbs in Latin have many meanings) "to do". "Ego bene ago" literally means "I well do" ("I am doing well"), but since "ago" is conjugated for the first person, "ego" may be dropped since it's already implied.
because when you ask someone "how do you feel?" you are asking them if they're well or sick. you're asking "how are you?" when you reply "sentio bene", that doesn't answer that question. it means something like "i feel emotions efficiently" which is not really what someone expects to hear as a response to how you're doing.