Idk what to do..

Okey.. so for the past 3 weeks I've been feeling nauseous all the time. I can't even go for a car ride without feeling like imma throw up... I've taken 3 different pregnancy test and they all came out negative.. and now for the past week the lower part of my belly up to my belly button and a little above is getting firm.. My period is suppose to start in 3 days but I haven't even cramped yet and I always cramp before it starts... I don't know what to do.. should I go to the doctor..??