Charging issues! Puzzling problem??

I have a early 90's Toyota corrolla. The alternator belt has been squealing for quite a while now but haven't had any issues until recently. The past week or so, the battery light has come on from time to time and it is obvious that there is a charging issue when the light is on. Most of the time the car drives fine with no warning lights on. Then last night the car died on me. Went back to move the car in the morning and it started right up. No warning lights and drove it home. I figured it is a bad alternator. But I checked the tension on the belt, which seems ok (maybe slightly loose but not much if any). I also did a voltage check on the battery. Read ~12.2 with the car off, then ~13.8 with the car running. However when the car is squealing, the voltage reads ~12.0. Obviously the alternator is not working to full capacity when the car is squealing which I would assume is a loose belt - but seems to be working fine as long as the belt is not 'slipping'. Funny thing is, when the battery light comes on, the car is never making the squealing noise, and it was not making the noise when it died on me. I began wondering what changed between the time that it has been squealing but never had charging issues and recently with all the charging issues. Only thing I can think about is that we hit a large, deep pothole in a construction zone at highway speeds with a full car. Was a pretty violent and hard hit. The first time the light came on was within an hour of that incident on the same drive. Any ideas what could be the problem??