Shrimp allergy, lip blisters, not allergic to lobster or crab?

I'm 14 almost 15.
As an Asian, my family cooked a lot of shrimp dishes and I never had a problem with eating them. At age 11, i had an allergy to shrimp. My lip was tingly and itchy. Then I started getting blisters on my lip. Specifically, they were 2 pretty big pouches, a little bigger than the home button on apple products (iPhone), on the left and right corners of my upper lip. It happened with smaller blisters at first, then I tried eating shrimp a week later and that happened, which is how I'm sure it was the shimp I'm allergic to.
I can eat lobster, crab, and other shellfish with no problem. What is it in shrimps that makes this happen?
After not eating shrimp for almost 4 years, I really wanted to know if I'm still allergic to shrimp. I had a small piece, the size of a pinky nail, and nothing happened. Also, I can eat dishes with shrimp, just not the actual pieces of shrimp.
Would it be safe for me to start eating shrimp again or should I wait longer?
If you had or have an allergy to shrimp, how long have you had it (or how long until you weren't allergic anymore)?

I'm allergic to shrimp and Scallops, but can eat Lobster, Crab, and other shellfish. It is just something your body rejects. Yes, after years of not eating it, you can temporarily eat it again. But don't go overboard or the allergies will return. (Happened to me) If you want to eat shrimp don't do it more than twice a week.

Aw sucks