My lumineers already stained after a few days?

When lumineers are placed, they are cemented on with a composite resin cement. The cement can stain if you drink tea, smoke, or drink red wine. The lumineers are placed on the surface of your tooth and do not go in between the teeth.  This allows a seam to show on the front of the tooth. This is one of the less desirable things about no prep lumineers. 

If they are lumineers, then they are porcelain and not composite.  Porcelain is highly resistant to staining.  I have been placing porcelain veneers since they were first introduced to dentistry back in 1985 and in all those years I have only seen one veneer patient get any markable staining, and that is because he smoked cigars.  Once I showed him how to properly clean his veneers, that visible staining no longer occurred.You may be seeing root areas that were not covered by the veneers.  This was commonly seen in older veneers due to the preparation design that was first taught.  Nowadays, most cosmetic dentists prepare the teeth a little differently, covering over more of the roots and teeth, eliminating "the show" of the darker underlying teeth.It would be difficult to comment with any accuracy exactly what might be happening in your case without actually seeing your veneers.  Thus, I suggest you first go back to your dentist, and if you aren't satisfied with the answer and any resolution, to seek a few opinions from other cosmetic dentists.