Arimidex and waking in night with heart beating

Have been on Arimidex for nearly 3 years and still waking at least 3 times in the night sweating. Recently I have noticed that I wake up sometimes to find my heart beating fast and wondered if this was a side effect and if it had happened to anyone else.

yep they changed me to letrozole and thats a bugger to so i am now not on anything(as of last thurs) so will see if i can move without pain
sue