Should I Tell My Girlfriend About These Letters?

I’m a 17-year-old guy, and I’ve been in a long-distance relationship with my girlfriend (also 17) since February. We’re both in high school. This term, I’ve gotten four letters from different girls—two anonymous, and two from people I know. One of the letters was from my ex, who said she wants to get back together. I implied that wouldn’t work because I’m with someone else, and since then, she hasn’t talked to me. The other letter was from a long-time friend, who confessed feelings for me.

I haven’t told my girlfriend about any of this. Part of me feels like she doesn’t need to know since I already rejected them, but another part of me thinks it might be better to be open about it. We’re long-distance, so communication is key, but I’m not sure if telling her would cause unnecessary stress. Is it wrong to keep it to myself, or would it be worse to bring it up?

EDIT: I'm in a school that doesn't allow phone's, it's a day school though so letters are how we communicate at school... I didn't mean like letters you send through a postal service.

UPDATE: I told her a few days after posting this..... I didn't say it in a manner that makes it seem like a big deal so she wouldn't think about it too much. We didn't really dwell on it we just talked about it in less than 8 sentences and it was gone. No big deal.