headphone volume lower than normal

This has been happening to my phone for the past few updates.

When my phone is plugged into the auxiliary cord in my car (basically the same thing as having headphones in) the volume is really low on my iPod. The only way the volume is fixed and goes back to the regular volume is if I get a text, a notifications, etc. This only happens with the iPod. I noticed it did NOT happen on Spotify. It also sometimes fixes itself if I restart my phone. But this is extremely irritating and I can't figure out how to stop it. i have to have the volume all the way up in my car, and then if I get a texted, it gets REALLY loud and it's quite startling. I've tried to play with settings and it hasn't worked. Has anyone else had a problem like this?