I'm on the depo shot and my nipples are sore the past few days?

I just went for my second depo shot today and I was not late getting it.....but a few days ago my nipples got sore and still are? What could this mean? I know this can be a sign of your period starting and pregnancy but neither of those are likely....I haven't had a period since little after I started the depo shot. Thanks in advance to anyone who can help.... I did see where someone said something about if you don't wear a bra your clothes rubbing them can cause them to be sore and that could be cause when I am home I don't wear one....I just don't remember this happenin before even when I had a period