Nausea and pressing on bladder

For about 3 or 4 days I've been feeling sick. I started feeling really sick on Wednesday night 2am then for two hours I've been going toilet because of that, around 4am I actually throw up *not much*. Since then I haven't been feeling any better. Also apparently a lot of people have stomach flu now but I'm not sure about that. 

Around 3-4 days before this I had sex with my boyfriend (one day before my period finished, I was off my pill *birth control*) he never  ejaculated inside. My bleeding was kind of ending on that day but few hours after sex I was bleeding a little bit again. (we have been together for 10 months and we're both clean, no infection, diseases or w/e) 

I really feel sick and I get this feeling like something is pressing on my bladder (I usually get this feeling when I feel sick) So basically > nausea, pressing on bladder and feeling like throwing up pretty much WHOLE day,

it doesn't really get better and I feel weak all the time + I have no appetite I even feel kind of sick when I think about eating something. 

Is it just stomach flu or what ? 
Reading all posts and stuff made me scared that I might be pregnant which I don't want yet, I'm not even 18 yet (in 4 months). > m <

k4lady