How can I get my girlfriend to be open with me?

My girlfriend and I have been together for almost 7 months, and up until the past week things have been fantastic. But recently she's been very closed off. She's always kept her feelings inside. But now I can start to see that whatever it is that she's keeping inside is starting to affect her. She's irritable all the time and she won't pick up my phone calls. I don't want to force her into opening up because I've learned my lesson on that subject. But I want to hold on to this girl and I just want things to be okay between us again, because at this point I feel uneasy and it's hard for me to focus on anything because my mind returns to thinking about what could be wrong with her. I know that this is a lot for me to handle and it's really bringing me down but the last thing I want to do is break up with her. I want to stick by her and get through this tough time. Any advice?