Couple of things... for future ref if you throw up within about 4 hours of taking the emergency contraceptive, it might not be effective, and you might need to take another.
In your case it sounds like it's still done something.
Your period pains are likely caused by your ovaries releasing an egg (particularly if you get the pain in your sides a few days before your period starts every month). This time, it is unlikely you've just released an egg as it's not been too long since the last time, and so you will not have your period pain. This is nothing to worry about and is normal for your circumstances.
Through the month your uterine lining builds up, if you're not pregnant you loose this lining and it comes out as your period. If it's not been long since your last period, there may well just not be quite as much there. So your period might be lighter and seem different.
In a nutshell, it sounds like you've not got anything to worry about.