Shrimp became mushy/grimy...too much lime juice or bad shrimp?

I made Papaya Salad with Shrimp. I used julienned green papaya, carrot, cilanto, mint and poached halved shrimp. The dressing is made of fresh-squeezed lime juice, fish sauce and sugar. I made it early this morning and the shrimp has now became mushy and grimy...totally lost its original snap. Is it because I left it too long in the dressing? I don't recall this happening before...