So I've been having this problem since yesterday, it's not a bad problem but I want to stop it incase it get worse.
At random times my phone will restart itself, but before it does it shows a message which I can't read (I think it said something with Temperature but I don't remember but if it is, it's never been a problem and I don't have much CPU usage apps anyways. My old phone was the same and had a lot more apps and never had this problem) because it shuts off as soon as that is said. Then it'll restart once of twice and be back to normal. I have no clue why it did this, maybe because I dropped it but it's in a thick phone case.
UPDATE: It happened just now when I got up and moved
I don't think it's a boot loop either because it gets to my lock-screen and allows me to do things, but the moment I walk and put it in my pocket it'll restart. I don't feel like buying a new phone when this phone is pretty new anyways.
Go to Settings/General/Battery. Tap the graph to get to the next graph. Look for a sudden drop to 0, followed by a sudden (not as sudden) rise back to what it was. If you see that, the battery is throwing a dendrite (a chemical thing in lithium batteries - possibly caused by the drop) and has to be replaced.
If not, maybe dropping it broke something else inside the phone. It's surprising how a drop on one corner can break something at another place. (The part under stress [caused during the manufacturing process] is usually what breaks, regardless of where it is in the phone.)
It is overheating. I had the same problem with my LG L34C. Never did get it fixed, I just stopped putting it in my jeans pocket. Mine would do it especially if it was in my front pocket while sitting down.