What Are The Side Effects Of Emergency Contraceptive Pills?

Hi, I took the first pill from my new packet of contraceptive pills yesterday morning and forgot to pack them and I'm at my boyfriends for a week and I don't live close so can't go back to get them. I'm not sure what to do now. Will that one pill affect me and make my period start again even though I've just finished or will I be ok? I know I can by them online from lloyds pharmacy but I don't have £30 to spare and don't need 3 months worth of pills. What should I do?