I actually just had something similar happen to me yesterday.
When I was at work, the phone died. So on my way home, I had it plugged into a USB charger in my car. I have a 1 hours commute home.
When I arrived at home, and unplugged the phone, I was surprised to hear the "battery low" alert. So I got inside, and plugged it into a wall charger (Kindle Fire charger.)
I woke up this morning, and when I took the phone off the charger, it still said battery is critically low. So I powered down the phone, removed the battery and waited 1 minute. When I plugged it back in, it no longer had the critical battery warning, but it now showed a full charge with the heart icon over the battery.
I went into the battery saver settings, and both settings were checked. I unchecked "battery saver mode now" and the heart went away. Then the phone showed 98% charged.
Having had a WP7/7.5 device for well over a year now, this was, by far the most awkward little bug I've encountered.
I guess my only suggestion would be pull the battery for 30 seconds or longer, then turn it back on and check all battery saver settings over. Good luck to you.