Should a concrete slab slope toward my house?
The slab upon which the addition is to be built should normally be level and without a slope to it - (a garage floor may be different). In addition the final grading of any surrounding dirt coming up to the slab/foundation should be BELOW the slab level and should slope away and down as it gets farther from the edge of the new house line.

The contractor that placed the slab is probably the wrong person to be getting assurances from regarding whether the non-level slab is OK. If they messed up the slab they are probably going to try to convince you that it is OK and try to proceed to build on top of it!! Get a formal opinion from 3rd party or your building inspector.

During the construction phase a new addition slab that is built up even with an existing floor line is likely to be susceptible to rain water coming in under the existing wall base plates even when the slab would be level. A good contractor would probably make allowances to place plastic up on the existing outside walls that drapes down over the new slab and out some 6 to 10 feet to help keep rain water from collecting up near the place where the new slab joins up against the existing wall area. If your slab with it's slope toward the existing house is a legitimate design then it seems to me that such temporary measure may be even more important. However I suspect that the slope is completely improper in this case for a bedroom addition and the fact that the contractor did not take steps to keep rain water from running into your house speaks volumes about the poor quality of the workmanship being done on your project.