My breasts are tender and slightly brown but came on my period.

Pregnancy test: Breast tenderness and darkening of the areola can be signs of pregnancy. Breast tenderness can arise for other reasons. Women can have a period after getting pregnant, but it is usually somewhat different than their normal (shorter duration, lighter, spotting, etc.) Having vaginal bleeding at the usual time of a women's period does not preclude pregnancy. I'd check a pregnancy test.