Electrical problems immediately after getting car back from shop

Today I picked up my car from the mechanic. This is the first time I had visited this mechanic, since I hadn't found a mechanic after I moved. I just had some simple stuff done - belts replaced and transmission fluid/filter changed. Normally, I would have done at least the belts myself, but I'm a law student living in a small apartment in the city and don't really have the time or space anymore.

Anyway, I got my car back this afternoon. I jumped in and started driving, and it was fine. However, as soon as I got to where I was going and shifted into park and sat there for a moment, the "check engine soon" light popped on, and my car began to periodically shiver. Each time it shivered, all of the electrical flickers. Headlights, dash lights, everything. It does this any time it sits for a few seconds either in park or in neutral. Otherwise, it starts up just fine, and the light, shiver, and flicker I described are the only things its doing.

Am I right in guessing this is an alternator problem? Also, should I be worried about the mechanic here? Did they mess something up when they changed the belt on the alternator? Or is it likely just something that comes along with throwing a new belt on an old alternator? I had never had any kind of problem like this before. For reference, I drive a 2003 Mitsubishi Lancer with about 95,000 miles on it.

I apologize if this isn't the place to turn for advice, but I wasn't sure where else to turn. Any advice you can offer is very much appreciated.

EDIT: SOLVED - Turns out several of you were just about right. A rat climbed in an chewed on some wires and other stuff. Turns out it was a coincidence (of course, the rat may have chewed it there: I left it there overnight). This caused it to be having problem with a lean misfire. The mechanic just replaced a couple of wires. It's fixed and didn't really cost anything. He dealt with me really frankly. To be honest it was pretty refreshing, especially after some stuff I had dealt with at other places. I have had some bad luck in the past, but this one had a pretty happy ending. Thanks everyone for your help.