Omeprazole is also meant to be taken daily. If you are using the same hit or miss dosing as your Celexa, you may have your answer. Some antibiotics carry a warning to wait at least 2 hours before taking an acid reducer. Perhaps your Celexa does as well, and that warning would be on the label. Have you, by any chance, read it?
Greenlydia
I take Omeprazole first thing in the morning and was told to take nothing else for an hour. Are you taking them at the same time? If so, you may want to take them an hour apart. The Omeprazol, being an acid reducer, may have interfered with the absorption of the Celexa.
This is just a guess as to why you had an attack though.