I'm about to turn 18 and haven't exercised in years and after some pushups+situps the past few days I finally went on a 25 minute walk and when I got back I started getting chest pain. I had them when I woke up in addition to a rapid pulse and shortness of breath. I really want to lose weight but I don't want to go back out there if I have a heart problem. (had an EKG and Chest X-Ray 4-5 months ago which was fine)

Any tips?