Celexa and Lexapro are exactly the same active drug. The chemical in Celexa, citalopram, is made up of two mirror image isomers, R-citalopram and S-citalopram - aka escitalopram. Only the 'S' form is active, with the 'R' version being about 50 times less potent. Lexapro, escitalopram, is the more refined version of citalopram, consisting of only the 'S' isomer. While R-citalopram is mostly inactive, it does have subtle effects which some notice, but others don't, however, the side-effects should be the same.
Both Celexa and Lexapro can cause a temporary increase in both anxiety and insomnia. This is a side effect of the medications, caused by an increase in serotonin levels. Your doctor has started you on a non-therapeutic dose, 5 mg, and this might help keep the side effects away. 10 mg is the lowest therapeutic dose for Lexapro (20 mg is the lowest therapeutic dose for Celexa).
If I can answer any other questions, or be of any help, let me know. The very best to you, JC!... Chuck