Emergency contraceptives (EC's) are pretty notorious for delaying your following period. You can always do a pregnancy test. Use the very first void (pee) of the morning as it will have the highest concentration of hormones. If it is negative, more than likey it is just the emergency contraception interfering with your cycle. It can also make you feel bloated, nauseated, make your breasts tender, headaches, cramps, etc. You may not have any side effects. Your cycle should straighten itself out in a cycle or two and be back to normal. There is a 5-30% chance for pregnancy, even with the EC pill depending on where you were in your cycle and you are correct in thinking that it is difficult to know when you have ovulated and that fertility calendars and apps are not very accurate but just guesstimates based on averages. Odds are best that you will be fine but you may want to consider getting on a more reliable birth control method. Condoms are not very reliable and you should always use spermicides with them in case of slippage or breakage. Best of all, the most reliable methods are the hormonal methods like birth control pills, Nuva Ring, the "patch", Depo-Provera (or the "shot"), Implanon/Nexplanon etc. These methods have a failure rate of 1% or less when used perfectly and about 1-2% failure rate with average use. Methods like oral contraceptives (known as the "Pill" or birth control pills), Nuva Ring and the "patch" depend on your consistency of use. If you miss pills or dont apply the patch or ring like you are supposed to the efficacy rate drops. Birth control pills taken regularly are a LOT more reliable than condoms or EC's. I'll take a 99.9% efficacy rate over an 80-95% one (like EC's) anytime! It is easy to get onto birth control. Many areas have family planning clinics for reduced costs and in some cases free or by donation only. If you are a student, often times these clinics are free. Another route is to see a gynecologist if you have health insurance. Even if you are underage, there are privacy laws for birth control so if you tell the physician not to tell your parent then they cannot (unless you are underage and you report that you have been raped) If you have a close relationship with your mother, talk to her about taking you to see a Dr or talk to another adult you can trust (an older sister or aunt or cousin etc). Many family planning clinics do NOT require you to have adult permission even if you are underage. They will examine you and prescribe the method that you feel is the correct one for your lifestyle. Once you are on reliable birth control you will have a lot less worry and stress! Good Luck!