Someone is remotely accessing my iPhone. What should I do?

During the past month I have noticed that my iPhone has been acting strangely. When sitting at my desk last week, my iPhone was on the desk and not being used. I wasn't close to anyone at the time and after a few moments of not using it, the screen lit up, the lock screen automatically slided across and my pass code was remotely entered. Before I knew it my home screen appeared. A minute later, I grabbed the phone and switched it off. Then, just this morning. I left my phone on the desk at home and went downstairs. There was nobody else in my home at the time this happened. I locked my iPhone and earlier in the morning I closed my apps and checked that my volume was muted and no music was playing. When I returned, I tapped the home button on my phone to log in and use it and an album cover appears and the music icons appeared in the lock screen as if I had began playing then left the phone. The song was about 2 minutes in when I accessed the iPhone. I find this completly strange and very concerning.

It seems obvious to me that someone is remotely accessing my iPhone. I would like to ask for some advice as to what I can so. I see this as a major security threat, and if someone is remotely accessing my iPhone. They could be accessing my emails, texts, etc which poses an even bigger threat.

I am the sole user of my iTunes account and nobody else knows my passcode as I change it on a regular basis. I also have an individual iCloud account and nobody else has access to it.

Thanks in advance for your help