Why do i feel sick all the time?

Since Sunday, I've been feelign really ill as in wanting to throw up all the time :s:

Whenever I'm hungry, i can't eat because of this sickly feeling. I don't know why and its annoying me! The feeling is not like a queasy stomache but more like tingly, gagging feeling in the throat. I havn't been eating properly for the past few days and i could only stomach some salad and a bowl of cereal each day...
Urgh...even as i type this message, i still got that gagging feeling in my throat!

The only reason i can think of was that my mam cooked some crab on sunday and as soon as i stepped into the kitchen and smelt the aroma, i felt really sick coz im violently allergic to crustaceans. Could that be the reason? Kinda like a three day hangover or something after a night out of excessive boozing? Any one with similar experiences? how can i get rid of this sickly feeling!? :argh: