My mother was convinced that she had just become unfit and that was why she was getting breathless every time she had to walk up a hill. She started getting little chest pains as well, which she attributed to the fitness thing as well.
Eventually it got so bad that I was able to convince her to go and see a doctor and it turned out that her heart wasn't beating properly. She ended up having to get a pacemaker, and the doctor told her recently that if she hadn't got it, she would have died by now, because her heart was so damaged that it probably can't beat by itself anymore.
It wasn't anything she did to it, either. They think she just caught a virus or something. But it does show that it's a good idea to go and get a check up, even if you tend to just blame yourself whenever your health seems a bit off. Chest pain and difficulty breathing are a big deal.