Hey hon
If you were buying your previous clothes from Evans or similar places then remember they are cut much larger and to totally different "shape" than clothes from non "plus size" shops. So an 18 in Evans will be totally different than an 18 in Dorothy Perkins. That may be why you are find this wierd confusion with sizes. It's then further complicated by the fact that each clothes chain will size their clothes differently even between trend lines within the store!!
So I know it must be disheartening but focus on the inches and remember we all lose weight from different parts of our bodies in different orders. Rumbly loses it from her face, one girl lost 3 shoe sizes before she dropped a trouser size, I've also lost weight from my feet and my neck. I've lost 6 inches from my hips but only 2 from my bust.
You just have to keep going and trust the plan. xx