Periods are odd things, the slightest change in your lifestyle, hormones-whatever and they get knocked a bit, theres no such thing as a regular period!
How long have you been on this diet? I do know nutrition changes do affect your cycle- so try not to worry, as the more stressed the later it might get lol
Give it a week and take a pregnancy test (if you're sexually active) if within 8-10 weeks theres been no sign of a period go see your GP
- btw don't take this as medical advice lol