Singulair Heart Palpatations, Asthma Attack, Short of Breath, Chest Pains, Mother and Daughter

My 11 year old daughter had been seen by a doctor to control coughing and wheezing brought on, I believe, by allergies. When she was 5 or 6 years old, she had an asthma attack and it happened again this year when we could not control the coughing and she was short of breath. That is when Singulair was introduced to her. about 4 weeks after taking medication, her heart started beating irregularly (called heart palpitations), and she was experiencing chest pains. (very frightening for mother and daughter). The Coughing subsided, and she was feeling good prior to the last week, when this started happening. After all the researching, the minor side effect is palpitations, however, how healthy is this and what else is going to pop up unexpectedly?