Usually my period is very predictable, but it was 3 days late this month. could celexa (citalopram) have caused that?

Unlikely: Unlikely. There are animal studies indicating Celexa (citalopram) may improve stress-induced missed periods. If concerned discuss with personal physician.

Could be: Antidepressants can have a variety of minor hormonal side effects. Keep an eye on your calendar for another couple of months and see if it persists. It's not a big deal regarding your reproductivity although the timing issue may be a nuisance for you.