iPhone 6 consistently turning off?

So I just updated to ios 8.4, and now every time I have been off my phone, when I turn it on again, nothing is working. I have no idea if this is because of a bug with ios 8.4, because I found other people had trouble with it on older software. I found this temporary fix where you could hold in the home button and the sleep/wake button for about 20 seconds, and then it would turn on.

The first time this happened was in the morning, after I woke up. I did what they said, and when I got it to turn on it was at like 1%. I immediately plugged it in and everything seemed fine. Even though I had it at 64% last night (didn't charge it over night) I thought to my self it was just a bug or an app that took battery or something.

But then it happened again! When I had been off my phone for a while it was the same thing. But when I did the 20-second trick it hadn't drained battery? As always before I did the 20-second trick I checked that the buttons were unresponsive, so it was the same problem.

Now the third time it happened was when I started to think it was a problem. Everything was the same as the second time (battery wasn't drained).

It's getting really annoying, and I have no other clue than that it has to be ios 8.4's fault.

Other reasons may be:

• Moved to a new country and changed SIM card.
• Backed up my iPhone (after first encounter with the bug).
• Deleted a lot of apps because too little storage to backup.

Is my phone trying to protect itself in some way? I haven't changed any settings, and it hasn't been laying in the hot/cold.

It's getting really annoying to have to do this every time I want to use my phone after a long break! Anyone having the same problem after updating to ios 8.4, or even better, having a permanent fix?

Any help would be much appreciated!

Thanks,

Magnus