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Hello and welcome to my blog today.
Its 
1. Tight Clothes.
Tight clothes causes friction on the boobs thereby causing you to feel pains in your nipples. Tight bras, polos, shirts and even tight towel or wrapper tied on ur chest also causes you to feel pains in your boobs.
2. Long hours of exercise.
Long hours of exercise also contributes to nipple pain. I've experience this before. Sometimes when I do my squats and lounges, I feel pain after i stop.
3. Pregnancy.
A pregnant woman will most likely experience nipple pain because of lactation. When the milk starts forming inside your boobs, you will most likely experience nipple pain as a pregnant woman. Some people will know they are pregnant if they press their nipple and feel pain that's when they've not seen their period though.
4. Infection.
Infections are also causes of nipple pain. There's a case of infection of the nipple caused by Candida Albicans which occurs as a result of damaged of the surrounding tissues or when a person has had fungal infections before now.
5. Allergies.
Allergies can also cause nipple pain. It could be due to a reaction from body lotion, perfumes, soap, body wash, detergent, shaving cream, fabrics e.t.c.
6. Sexual activity.
Sometimes during sexual contact, sucking or pressing of the breast could lead to soreness. You could also feel nipple pain during sexual contact due to body friction which leads to a temporary pain giving time to the nipple to heal.
7. Menstruation.
Menses could also cause nipple pain as a result of changes in the hormones of the normal monthly cycle of a woman's menstruation. This pain could start fays before the period of a woman but if the pains persist, the woman is adciced to visit a doctor for proper health examination.
8. Cancer.
Cancer is another cause of nipple pain. This is one of the symptoms of Cancer. Nipple pain caused by cancer can lead to the infection of only one breast and nipple. Other symtoms of breast Cancer includes, yellowish discharge from the nipple, reddish nipple, flattened or inverted nipple, itching of the nipple etc.
It is advised to see a doctor when you feel unusual pain in your nipple.
Thanks for reading, have a lovely day and God bless you. Please do well to upvote, comment and resteem.