Why Is My Vagina Itchy and Discharging Yellow?

If you had protected sex and have missed your period, there could be many reasons for a missed period other than pregnancy. Stress, changes in weight, illness, hormonal imbalances, and various medical conditions can all affect your menstrual cycle. Please get a test done to confirm about pregnancy.

FAST PERIODS could mean a HORMONAL IMBALANCE....STRESS, weight loss, or PCOS could be the reason...CONSULT a GYNECOLOGIST to rule out other causes... KEEP TRACK of your menstrual cycle using a period calendar... MAINTAIN a HEALTHY WEIGHT, rest well, and EAT healthily...TRY YOGA or MEDITATION to manage stress....

In this case there's no chance of pregnancy.

When the contraceptive pill is taken correctly its effective in preventing pregnancy.. but there is still a small chance of pregnancy even if ejaculation doesn't occur inside the vagina. Take a test to confirm

After ectopic surgery you can try to conceive after 3 months 

You’re experiencing strong nausea, a backache, and lower abdominal pain, and you’re wondering if you might be pregnant. These symptoms are common in early pregnancy, especially if you’ve had unprotected sex recently. However, they could also be caused by other digestive issues. The best way to find out is to take a pregnancy test. This will help confirm whether you’re pregnant or if something else is causing your symptoms.

Menstrual bleeding patterns often change when hormonal tablets are administered. But, if your menstruation continues for too long than usual then medical help of a gynecologists is required.

While it's less likely to conceive right after having your period, it's still possible to become pregnant at any point in your menstrual cycle, including during the days immediately after your period. 
If you've had unprotected intercourse and missed a period, it's important to consider the possibility of pregnancy and take a pregnancy test to confirm.