Weak, shakey legs + severe stomach cramping and nausea?

I have had sever stomach cramping and nausea for 5 days now and it shows no sign of getting less painfull. i didnt know what was the cause because i havent eaten anything unusual to what i normaly eat. also i do not feel sick all the time, i get sudden waves of nausea and i feel i need to throw up but i dont. today every time im stand up my legs are very shakey and i have to sit back down again because i feel that my legs are going to buckle. i would be very grateful for any serious ideas and contributions because i do not know what is wrong. thankyou in advance.