What could be wrong with my car?

Not too long ago, the timing belt went on my 1991 Plymouth Acclaim. I replaced it myself, making sure to line it up to TDC. About a month after this, one day while starting my car, i noticed it was running much rougher than it had been since the repair. After this happened, when i first start driving the car, before it warms up, it has very poor power and makes popping sounds coming from the engine/exhaust and seems to be getting worse. Is it possible the timing slipped? If i fix the timing would my problems be solved? Or are my valves bent to the point where i need a new car?