Could really use some opinions on what to do
So I started dating this girl a few months back, we have known each other for a year or two but were on a more of a first name basis for that time... had a fun night out with a bunch of friends one night and I worked it up to ask her out... and now that we have been dating a little while I can't help but get jealous because one of her best friends is a guy, in fact a guy who about a month ago didn't tell her how he liked her and everything... but now there still friends and I don't know, it may just be me overthinking but now she seems to not be really putting any effort into keeping the realationship going, and they go out to lunch and stuff together. Plus she has kept notes that he has written her from way back, nothing romantic I don't think... but still the fact that she has kept them when she knows he wrote them before telling her that he had feelings...
Should I be jealous/worried about anything, I really like this girl but its driving me crazy when she says stuff like, I'm working today so we can't hangout anymore sorry, but then goes and has lunch with him the next day before they both go to work...
Thank-you for reading and sorry in adavance about spelling