31 weeks pregnant, why cant I stop throwing everything I eat or drink up?

I am 31 weeks pregnant, due September 5th and for the past month and a half I throw up atleast 10 times a day.. I have even been throwing up just plain water, but according to the doctor im not dehyderated. I dont see how. The doctor has tried me on 4 different pills for nausea but nothing seems to work. I have lost 2 pounds in the past month, when I thought you were suppose to gain weight come the third trimester. My doctor says everything is fine and normal, but I still dont find it right to be throwing everything up and losing weight.. I have also been having bad pains in my pelvic area almost like someone is stabbing me, which has been happening frequently for the past few days. I dont know what it could be since I am only 31 weeks I know it isnt labor pains already.. Any ideas.?