Girl dance symptoms and conditions

My 9 year old daughter has been on Singulair for almost 6 months. Her asthma symptoms have completely gone away. She no longer has her night time cough, post nasal drip, and frequent colds due to allergies. However, she has gained almost 10 pounds, has been tired and has complained of sore muscles. She has had night mares and leg cramps at night. She has also had frequent mood swings. I thought this was just due to her age. She used to be a sweet, kind, patient girl most of the time, and she has been quick to have temper tantrums, cry and/or yell at us over the silliest things. She is a very active girl- dance, soccer, swimming, etc. and she eats healthy foods for the most part. I could not understand why her face was looking so bloated and her pants were all getting too tight all in such a short amount of time. We are taking her off of this medication immediately!