I have one child myself, and having gone through the whole pregnancy thing i'm going to tell u to go and get tested. For mothers in the first trimester, fatigue, breast tenderness, nausia, are the main symptoms. U may just be very stressed out...Although I still am encouraging u to go and get tested.
Do u want to be pregnant??? If u don't I suggest u talk to a dr and try to come up with a good birth control method that works for u. There are so many different methods out there to prevent std's and pregnancy. If ur going to have sex, it will be ur responsiblity to make sure u take care of ur body, and is cautious. Be responsible.
Hope that helps u some.