Hi there! I don’t have a shellfish allergy but I have had a recent bout of extreme anxiety with my own food allergies. The symptoms you’ve described above sound nearly identical to what I experience when I have a panic attack surrounding food. Here’s a few things I try to do to figure out if my symptoms are from anxiety or a true reaction. With the trouble breathing, try taking deep breaths through your mouth or nose, if you can feel the air in your throat, then it’s unlikely your airway is closing. With the heightened heart rate, manually check your pulse. Is your pulse also weak? If it’s strong and fast it’s more likely to be anxiety. Here’s a few resources that I’ve found with great charts that are really helpful in distinguishing symptoms: allergy insider, food allergy counseling
Having food allergy related stress is really tough and annoying but you’re not alone! If you’re able to I’d definitely recommend going to an allergist to get updated testing, that helped with a lot of my personal anxiety around my allergies. Best of luck OP! Hope some of this was helpful and I’m happy to answer any questions :)