Once you can cook small pieces of toast on a PIC, you will have catastrophic failure in short order. This I know from plugging the programming header in wrong. It worked for a while, drawing more current than usual, then quit. I'd suggest replacing it, and never connecting an I/O directly to a B+ line.Put a 1K resistor in series if necessary. If you are running at 3.3 volts and using a LiPo which can be charged to 4.2 volts, You will stress the diodes beyond their limit. If there is internal damage to the gate or protection diodes, it's a bad thing.