As you finally came, you fill the insides of the costume with your semen. However, as soon as it came in contact with the suit it absorbed it, activating itself as it began to fuse with your body.
You feel so pleased with yourself that you cannot do anything but lay down and watch. The mask combined to your head, turning it into an humanoid tiger, and your body started to bulk up in muscle, and the tiny furs pierced your skin and became your own fur.
Your girlfriend was still int he bathroom, watching you transform into a weretiger, and getting horny as it was happening. You can tell the same, as your new body was as powerful as it was impressive. However, there was something about the costume none of you considered....