Which side of your chest?
If its left and center its your heart. Usually when I get a pain its in my right side and while its been there, its hardly stopped me from being a long distance runner.
Your pain would seem to be obviously due to high blood pressure. Running naturally increases your blood pressure anyway, so that pain you're feel is most likely the increased stress on your heart. Not cool. See a doctor. Push any cardio workout too hard and you could wind up with serious complications.
A final note on running; For cardio I hate tread mills. I loathe and despise them. Bikes are alright, but you're main goal here is not to get into cardio shape.
Your main goal here seems to be weight loss and high blood pressure is causing complications for you. It sounds like you should start first with your diet before thinking about taking on any cardio due to the above considerations. After that just do your workout regularly for an extended period of time and your metabolism should pick up and do all that natural fat burning you're trying to get going.
-Cyrano