feeling sick but it won't come up even if i heave/gag?

[Jessica]

i have been feeling sick for about a month, it's like a flush of sickness quite often and then it will go, but it's been going on for about a month and recently this week i have began to heave/gag as if i'm going to be sick but nothing will come up, this has been going on for a week now and it seems to act more when i'm stood up. I'm not quite sure what to do about it so could anyone give me any advice or does anyone know what this is? I havent been to see my doctor yet, so if you think i should just write that down. thanks xxx