Pepto-bismol molar bismuth concentration
Your calculation is correct. You are not missing anything obvious. Your set up, which converts mL to L and then mg to mmol is correct and you get the correct units.

$$\require{cancel}
\mathrm{\dfrac{17.5\ \cancel{mg}}{\cancel{mL}}\times \dfrac{1000\ \cancel{mL}}{L}\times \dfrac{mmol}{364.10054\ \cancel{mg}} =48.2\ mM}$$

This concentration is not "too high". It is what it is. 48.2 mM is 0.0482 M, which is a pretty low concentration. 

For comparison, your 17.5 mg per mL is at most 2% by mass (low-balling the density of the suspension to be around 1). 1 M hydrochloric acid (which perhaps seems like a high concentration) is around 3% by mass. Concentrated HCl is 36% by mass and about 12 M.