If there's no apparent reason, then the possibilities are:

• There is a reason, but you can't tell what it is. Maybe you've learned to be nervous in some situations (the end of the month is a busy time at work, spring is the time when finals are, people have been visiting a lot and you haven't had time to yourself). You don't consciously remember any reason to be nervous, but you've been nervous in situations like this before and it's an unconscious reaction now.
• There really isn't a reason. We evolved to have feelings and emotions because they're useful ways to keep us out of trouble without having to constantly think about every problem rationally (and slowly). For an early primate it's good to be nervous when you come down from a tree or go to the watering hole even if you can't find any obvious reason to be. There's a slightly higher chance of things going wrong, so being a little nervous is helpful for the tiny chance you're going to have to react quickly. Being a little extra careful when there's no danger is a lot better than not being careful enough when there is. In our modern world we probably don't need to be nervous or anxious nearly as much, but we haven't had time to evolve away from those feelings.

Either way, when I start to feel nervous I try to remind myself not to be nervous about feeling nervous. It's a normal biological response and it's not really fully in my control and sometimes it's just going to go off for no reason. Sometimes I'm going to get really nervous over a little thing, but that's not really all that bad, it happens to everyone. As long as it's not keeping you from going out or doing things you like, just accept that it's something that happens because our bodies are messy bags of chemical goo that evolved for a hard and difficult life that doesn't really exist anymore - thankfully :)