Last night i have developed a sore and tender nipple. its slightly swollen. i applied antiseptic cream and a dressing but still very tender?

? infection: If there was an oral manipulation or biting of the area or any trauma there may be an early infection that will require antibiotics. Best to see a physician.

Get it checked : You are describing the beginning of an infection (swelling and soreness). For this you applied the right remedy. If the soreness resolves within 24 -48 hours you are fine. However if this is getting worse, or you develop additional symptoms like redness, drainage, pus, fever, and chills, then you are developing a more serious infection and need to be seen by a health professional right away.