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So since I got my HTC One m8 it never used too much power, or drained while charging until now. I have used my phone while charging and it did reduce charging or took longer to charge (obviously) but today it started using more power than before when I saw the power decreasing. I tried my old HTC charger from previous phone and its charging normally. What's the problem? And can I switch back to my charger w/o problem?
It's possible your M8's original charger has become faulty. When you're charging with it plugged into a wall outlet and you go to Settings>Battery, does it say Charging (AC) or Charging (USB)? If it says the latter, then either the cable or the wall plug may be faulty.
When in the outlet, it says AC
When you say that you tried your old HTC charger from a previous phone, do you mean the USB cable only, the wall plug only, or both?
If other chargers work normally, then it still sounds like something about the M8's charger went bad. You may just have to use a different charger (unless you want to buy a new OEM M8 charger, which will probably be the most expensive option).
I used both USB and adapter
It could be a defective charger, a defective connection in the cable, a defective (or mistreated) battery or something defective in the phone. There's no easy way of determining which from here. Your best bet is substitution. Substitute a known-good charger and cord (you can't easily change the battery). If that doesn't fix it, the phone has to go to the shop for further diagnosis. If that fixes it, substitute your cord or your charger (not both) and you'll know which is defective.
Also - if the CPU and GPU are running heavy because you're playing a really intensive game, the phone will be using more current than if you're staring at the homescreen or reading an email, so be sure to compare apples to apples. "It charged fine when I read emails months ago, but now it discharges when I play X" isn't very meaningful.
Since changing the charger and USB cable seemed to work, that's probably where the problem lies. USB cables are prone to going bad and cause charging issues, plus they are the cheapest option. I'd suspect and try replacing that first. You could also just use the other USB cable you already have on the M8 charger. If that works, you're good to go.
Thanks for all the suggestions
Some devices deplete the battery before it charges when its below 5%.