I Felt Nauseous For 1 Whole Week Now And It

I felt nauseous for 1 whole week now and it makes me lose some periods in class and it's going to affect my marks soon I have a vomit phobia and when I feel sick I can't get it off my head if I feel sick I have to go to the washroom like I don't have a choice. This is getting out of hand and is there a way to stop this? I want to stop missing classes