Mixing the two medication will make you drowsy for some time until your body adjusts to the medication combination. Also I would recommend asking your doctor to start "weening" you off the Ativan as you have been taking it for quite some time and most like your body has become dependent on it, (esp. at 3 times a day). See if it is possible to go gradually down to two (I would suggest the a.m. does to be removed first since you are experiencing some drowsiness with the Celexa).
Final note- if you continue to feel "weird" I would strongly recommend calling your doctor. - Muse