I developed my crustacean allergy while living in Houston and consuming more shrimp than I had in Dallas--it was so fresh.
You should go to a board certified allergist to get tested with a blood test (not just a skin prick test) to determine if you actually have an allergy.
If you do, continuing to eat them is very dangerous. Zantac (not Nexium) is actually an antihistamine. It could be that you've gotten some relief from that. But if you are allergic, you need to carry an Epipen (or AuviQ--a much smaller device) with you at all times. And if you are, you'll need to avoid crawdad boils. I know it's hard to do that--such a social thing.
But your life is worth it.
YOu should know that the way you react can change at any time. So just because you haven't had a sudden blood pressure drop, swollen/closing throat, and hives doesn't mean you won't in the future. Just because you haven't died from eating them yet doesn't mean you won't later.
But the first step (before eating any more) is seeing if this is actually an allergy.
If it is, you may have to worry about other things. I had my worst shellfish reaction when a restaurant used tongs to turn my steak that had been used to turn shrimp. I had eaten the very same thing two days earlier at this restaurant (I was there for work) and used the same exact language in ordering, letting my waitstaff know of my food allergy and that I had to avoid cross contact with shrimp, lobster and crab. But the chef made a mistake, and I paid for it with anaphylaxis.
I have also had reactions because my food was on the same grill surface that shrimp had been on (very few restaurants have a separate area for shellfish). A chef will say he can clean the surface, but that doesn't typically involve using something that will actually remove the shrimp protein. I also have to worry about what else has been fried in something I'm eating (tortilla chips are often fried in the same oil as shrimp).
You may not have to worry about that level of cross contact or cross contamination. I'm just letting you know what some people with a shellfish allergy have to consider when going out to eat.
Good luck with this. Glad you found us.