Hi there

It sounds as though the frequency of training is too low, and combined with the high intensity of the training, your muscles might never be fully adapting to the training. The reason for this stiffness, incidentally, is that your muscles are unaccustomed to the load on them, and they get very tiny tears, which then cause swelling and inflammation, which causes the pain. So the reason for pain is that the muscle is actually damaged. With training, that pain disappears, because the stronger muscle is less likely to get damaged. So that's why regular training will help overcome it.

Now it sounds as though you might be training each muscle group too infrequently, which means it never really adapts. Also, if your training is very intense, then all that happens is that the muscle gets slightly damaged, recovers, before you train again. What I would suggest is reducing the intensity (lift lighter weights or do fewer repetitions) and also try to train each muscle group twice a week. That shouldn't be too difficult to do, if you cut down on the number of reps and sets. You could, for example, combine arms and abs on one day, legs and abs the next, and arms and legs on the third. But it's important that you train much easier than you are now, for a couple of weeks, and see how it goes.

Good luck