98 Contour running on battery

I changed my alternator about 5 months ago and the car ran fine but recently, the the car has been running on the battery only. I get no indication (lights) like I did last time when the alternator was going out. I did the whole disconnect the negative terminal when running and the car dies. I put the battery in my truck with no problem and it seems to be working fine.
I had the car parts store do a test and it showed that the alternator was putting out 12-13 volts, but it registered negative amps (-22.6)
What could be causing this??

I would suggest a rebuilt alternator. Pull the alternator and have it bench tested and full fielded. Any alternator shop will check it out for you and probably fix it cheaper than a rebuilt.

I just had the battery and alternator tested at a different auto parts store. The battery is good, but the alternator is putting out 12.88 volts and less than 30 amps.

I was hoping I wouldn't have to change that thing. It's a pain in the rear.

OK, after going to three different auto places, the last one noticed that my battery light wasn't coming on when initially turning the key. He suggested to check the wiring schematics and find out if that's the issue.

Low and behold, the light runs of the second, smaller connector on the alternator. I went underneath and found that it had fallen off. I connected it, and the battery light now comes on.

After a short class for my daughter about checking for ALL the lights when first turning the car on, I sent her on her way.