There are two potential issues when you started to work for your boss.
1. It looks like you have more work load thean expected. The company needs more resources or more experienced person to do your job.
2. You work for another manager. There are conflict of insterst. For your boss, he want you to put his tasks in pririoty, of course.
My suggestion is to schedule a meeting with your boss, boss' boss and another manager to discuss the issues such as work loads, expectations, and pririoties. You need to collect the facts such as which projects you are doing, how long it takes, the resources you have etc.