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It's difficult to determine the exact cause without further info.. Stomach aches can have various causes which are unrelated factors like stress or dietary changes. If you're concerned about pregnancy then you may take a pregnancy test for confirmation.

Cervical polyps usually come back. Sometimes, one or more of the symptoms you experience are abnormal bleeding, pain, or spotting. The cause of this is not known with certainty, but it is most likely due to the changing hormone levels in the body, or prolonged infection that has not been cured. The polyp is often removed because it is usually problem-free. The doctor should closely monitor your health to know precisely everything is normal.

Periods can be delayed due to stress or changes in routines. Other factors include hormonal imbalances or a possible pregnancy. If the kit test resulted in a negative, consider consulting your gynecologist after waiting a while.

Having sex without protection on the second day of your period usually means there is a decreased chance of becoming pregnant. During this time, the egg is less likely to be ready for fertilization. However, it is not impossible to get pregnant so if you want to be extra safe, you might want to take emergency contraception within 72 hours just in case. 

If someone vomits everything they eat or drink, along with frequent headaches and pain in the back, this could be a sign of various things not just pregnancy. These symptoms may result from a stomach virus, food poisoning, or even migraines. Consult a healthcare provider for proper diagnosis and treatment.