Your comments warrant a couple more questions.
Was the shrimp already peeled and deveined, or did you do this yourself at home?
The reason I ask is something called STTP (Sodium Tri-Poly Phosphate)
Processors will use this chemical to absorb water into the shrimp, making it weigh more, which you buy per lb, not by the piece.
When the shell is left on, the STTP can't penetrate the shell, but some will be absorbed near the exposed 'throat meat'.
However, generally speaking, STTP won't cause mushiness unless an abusive amount was used (which is entirely possible) because processor mentality is "if a little is good, a lot has to be better"
By law, STTP is supposed to be labelled as an ingredient, but it rarely is.
Also, if you ever see the legs turning black (industry term is "Blueing"), this means the oxidization process has started. Shrimpers and processors like to use a little Sodium BiSulphite to prevent this from happening, especially with fresh product.
The Cat Man