This is totally normal, having girlfriend issues is something that many men(and women) have had to deal with at one point in their life. The first thing you should do is talk to her about it. Girls love to talk, but keep in mind that beating around the bush won't solve the problem. You need to sit down with her when you both are free of distractions so you can really listen and hear what she has to say.
The next most important thing to do is to ACTUALLY tell her what you're feeling too. All the while don't let things get about pointing the finger and blame, instead try and find common ground you both can understand. By reaching a compromise you both can get what you want out of the relationship, without one side feeling like they are doing all the work and getting neglected.
Next, you should now be able to tell if there are little things that just need some fine tuning, or if something bigger is going on here. Have the little problems popped up recently, or have they by and large always been there. If this is an ongoing problem then it may be easier to work out, people have natural highs and lows and this is nothing to worry about. But if this is something recent, has she began to act strangely in addition? Because this is could be the effect of something bigger. If you're sure that you haven't caused a rift between the two of you, and that she doesn't think otherwise then you may have a third party at work here. But so long as you have been good to her and she believes so I don't see why she wouldn't want to work through anything with you.
Every woman wants relatively few things in a man, other than the 'type' she looks for. She wants to feel safe, be cared for(loved, eventually) and trust in your fidelity. If you can meet and maintain these then you can't go wrong. Worst case scenario, things don't work out, it's not the end of the world-even though that's how it feels. Some relationships just don't work out, and it does more harm to both sides by trying to make it work anyway.
You have to remember that without failures we would never grow and evolve. More often than not as humans we have to make mistakes several times before finding something that works. So even if things were to go south you should do your best to not focus on what you did wrong, so much as what you did right. So that in your next relationship you may apply what you have learned to keep growing as a person and as a partner. The world is so full of possibility, and to focus on your shortcomings is just a waste of too much time.
The faster you learn to do your best to keep growing and to keep on keeping on the sooner you will be back into a situation you want to see yourself in. I hope this helps to keep you from having girlfriend issues!