This is another one of those things that varies from girl to girl. The first time I got my period I didn't notice any changes in my breasts. However, as I've gotten older I can say that sometimes they are way more tender than usual. Breasts grow at a different rate for everyone and I was surprised when mine continued to grow as an adult. I went from a B cup to a C cup in my early 30s and I had already been on my period for 15 years! During the time when you're actually on your period, you may or may not notice a bit of swelling or tenderness. Sometimes you might notice that you have the tenderness without the swelling. If your breasts are especially sensitive during this time, I would suggest trying a more comfortable bra. Perhaps something with more padding can help.
