Said "red lines" can be the beginnings of stretch marks;
If you've recently gone through some amount of "Large" weight gain [like gained more than 5-7 pounds at one time] the fresh stretch mark can appear red, and with time will fade to a soft pink.
Stretch marks are also a sign of puberty, when the breasts start growing.
It is normal for your breasts to get larger as you grow.
Swelling and tenderness of the breasts around ovulation time can be normal.
Dark purplish lines along the sides of your breasts could be a sign of various conditions, including stretch marks or changes in skin elasticity. If these lines appear suddenly, are accompanied by pain, or are changing in appearance, it's advisable to consult a healthcare professional for an evaluation. Generally, if there are no other symptoms, it may be a normal variation, especially if you've experienced changes in weight or hormonal fluctuations.
No.
When in doubt, take a pregnancy test. Usually the first sign of pregnancy is a missed period and sore breasts.
depends on when you started puberty.
Yes. As breasts grow, it is normal for them to be tender or a bit sore.
Yes.
Yes
yes
Yes
Yes, it is completely normal, especially if you are athletic.