It's normal for the phone to heat up being charged or when using it, even for social media. A phone is ONLY overheating if you get a message the phone needs to cool down before using it. When you use a phone to charge or do pretty much anything you're using electricity from the battery. And using electricity from a battery causes it to heat naturally. As to your battery health question, a drop of 3% in 5 months is actually very good. Generally, one can expect a drop in battery health of 1% (or so) per month. Every time you fully charge the battery, it's health declines, which is normal for batteries. ALL BATTERIES.