As you obviously appreciate now eating just 1500 calories is doomed to failure. You need to try as hard as possible to retain as much muscle as possible to keep your metabolism burning.
Also, as I've said, you will lose some water when you come off cycle. I wouldn't mind betting that half that 20lb is water. Point is, expect it and don't panic and think it's muscle going down the plughole and then eat too much to try and stop the loss. You'll just get fat.
Eat plenty of protein, don't shun healthy fats and manipulate your carbs for the energy to keep training but not spill over into fat gain. You'll get it right eventually.