Underexposure problem maybe? please help!

I bought a Canon AE-1 camera at a local camera store. They told me it was in perfect working condition and it was for the first month or so. I used it to take photos and it worked beautifully. But now not so much. I am in a photography class where I shoot and process my own film. The first time i did it it worked fine. But the other day i shot my film and when i looked at it, all the negatives were very underexposed, so i could vaguely see the images. except for the last frame, where it was a perfect negative. How could this happen? I did the battery check and it says its fine. My teacher suggested it could have been that i moved my ISO during the shooting until the last one? also when i adjust my f stop, the light meter does not change at all. could it be something with the light meter or the camera not reading the change in the f stop?