Thanks for your question. The answer depends a little on what you mean by "sinking."

If you mean that it is visibly sagging, especially if the sagging or body impression is already visible even when you're not on the mattress, then the answer is certainly no (this is not normal).

If you mean that your hips sink a little into the cushioning of the mattress, especially when you're on your side, then this could very well be totally normal. As a Plush mattress model, this mattress is designed to provide a lot of softness in its top cushioning layers, so it is no surprise that your hips (which are the heaviest part of the body) would sink into this cushioning a bit. For side sleepers or other people who like a lot of softness and/or cushion depth, this is generally considered a desirable feature. For other people this is an undesirable feature (though this does not mean its abnormal).

Either way, if you have a return policy and after another week or two you are not happy with the way the mattress feels or the way you are sleeping on it, you'll want to be sure to contact your retailer to understand your options in the way of a replacement.