have you checked the usb lead for damage/dirt?
you may need to open iphone and just check the battery connections are making good contact and screw is in (next to don't remove sticker)
what's your reception like? if it isn't as good as it was you may not have good contact between little gold antenna on top of motherboard (next to don't remove sticker) and metal bezel. if this is not in contact then phone uses all it's energy to maintain connection hence battery drain