Lymph node in breast???

Last week before I was supposed to get my period, my breasts were super tender. After a two days after my period started, my left breast was way more tender than my right. I also felt a bump in there that was tender. Now after not touching it for 2 days, it barely hurts & the node ? is much smaller (pea size). I'm 20 so I've been asking my mom about it but she keeps telling me is nothing. Do lymph nodes inflame like this? It's located on my left breast, on the left side, towards the armpit but not on the armpit (still on breast).