The pill is about 85% effective when taken within the first 72 hours.
Your period may be late, early, heavy, light... it depends how your body reacts to the hormones. But, if you're late or your period is extremely light and short, then go ahead and take a hpt just to be safe.
You may or may not have side effects. Some common ones are nausea, vomiting, headaches, breast tenderness, dizziness, fluid retention and irregular bleeding.
Because of the high dose of hormones, your periods may be a little crazy for up to three months.
Good luck!