lexapro side effect

Im on 20mg lex. for anxiety. Ive been on it for about 7weeks now and I take 10mg at night and 10 in the morning. when i take it at night, I will wake up exctly 3hrs after taking it and be in a panic. I feel the same way everytime it happens and cant remember much when I wake up. 3hrs after taking it in the morning I feel fine. Why is this happeneing, is it a side effect?