So, this could be a simple problem or much grander than I imagine.
I continually test the app I'm developing by connecting my iPhone 6 to my Mac and compiling/running my app on my iPhone. This process is ok and has no problems.
However, if I do not compile and run my app from Xcode on my iPhone for at least a few days, when I try to open my app on my iPhone (that has already been installed because, as many of you may know, when you compile and run an app from Xcode to a device, it installs it to that device), it crashes immediately.
I tried finding crash reports for this, but have failed to find any. I don't think any are being recorded for my app.
I have a feeling this is a code-signing issue, but I would just like to fix it before I do any sort of distribution.
TL;DR: I test my app by compiling it to my iPhone. If I don't do this after a few days and try to open my app directly from my iPhone, it crashes immediately.