Shrimp are crustaceans & mussels are mollusks, a different group of fish.
If the shrimp reaction was severe and the shrimp allergy proven, a skin test to mussels could help find out if eating them is risky. If you have already eaten mussels after the possible shrimp reaction, it is probably safe. Safest advice: get tested for both, maybe both will be negative. Challenge in the safety of dr office.