If you are taking pills regularly without missing even a single day then the chances of failure are rare.If one misses them for a single day and takes the double dose next day then also there are no chances of failure but missing for two consecutive days might cause failure.Cramping can be due to withdrawal bleeding at the end of the cycle.Nausea
is one of the most common side effect and weans off gradually.Take some antacid
and avoid spicy food.You can do a pregnancy
test in urine with kit available in the market so that you are more sure.
Best wishes and good luck