Lily Allen has a heart murmur.The 'Smile' singer has revealed her condition - an abnormal heartbeat sound often caused by a narrowing of the blood valves, or a hole in the heart - is the reason why she shed an astonishing 19lbs in just six weeks and has a new hardcore exercise regime. She said: "I found out I had a heart murmur about three months ago during a routine check-up. It was really scary."When they asked me how much exercise I did, I had to be honest and say hardly any."Now I make sure I work out three times a week. And I don't hold back - it's a hardcore routine in the gym."However, Lily stressed she has no plans to go too far with her diet and become a size zero.She added in an interview with Grazia magazine: "I really want to stress that I haven't done this because I want to become a 'Skinny Minnie' - I will never be like that."The 22-year-old star also revealed her murmur has even made her consider giving up her beloved cigarettes.Lily - who is dating Chemical Brothers star Ed Simons - added: "I was going pretty crazy back in the summer, drinking too much and partying loads. But this heart thing has made me cut back."I should also give up smoking, but I haven't quite managed that yet."