You're experiencing the normal transition from college life into full-time work life.
Where as classes used to be 45 minutes to 90 minutes, and then you could take a break, you are now faced with 8 hour workdays in which you have to be "on" for the whole time.
It's a personal productivity issue. What's happening is you will have a period where the days go slow and you won't be sure what to do with your time. Your company doesn't want to assign high-risk projects to you because you're new, so you will have "boring" assignments that don't take much time or attention.
You could ask your boss if there is any documentation you could read about the project. If there isn't any, ask if you can create them. This would give you a lot of learning into how the system works and would create documentation for future developers, which is useful.
You could also start reading the code to understand how it works. Draw pictures and ask questions, so that when higher risk projects do come, you will arrive at solutions quicker.
Another option still, is to buy programming books and read them in your down time.
Ultimately, this "boredom" issue will get solved with time. You will eventually be able to focus for longer periods of time and do more work within an 8 hour day, and find what works for you in terms of staying productive. In addition, you will get assigned responsibilities that take up your time.
Until then, you will struggle with how to use your time effectively. Everyone goes through it. Just make sure you're doing your work!