Weight Gain From Medication (Gleevec)?

I've been on Gleevec for CML since November of 2002. I've been steadily gaining weight and I believe it's due to the medication. I eat very healthy (lots of fresh spinach salads with fresh veggies, lean meat, fish), don't eat chips, sweets and other junk food, don't drink soda, drink about 1.5 liters of water in the morning at work and a liter of green tea in the afternoon at home. I figure my daily calorie intake is around 1000 calories a day. I used to exercise daily but there were no changes whatsoever and I got frustrated and stopped. I'm still active in my daily routine and I walk a lot. I'm getting frustrated that I seem to just be gaining and gaining. :-( What can I do? I can't stop taking Gleevec.