Dog experts please help.....?

Ok, i have a male boxer and a female pitbull, both sweet as can be. Well both of my dogs are under a year old so they are still babies to me. Well my pitbull went in heat, (her first one), because i didnt have money to get her fixed, but i want to. Well i went to Petsmart and bought one of those pet diapers for female dogs in heat. Ok i think she got knocked up somehow and im freaking out because i dont want her to ever have babies and she is way to young anyway. I think this because her are feeling like they are growing and look like it too. Could this be because of her heat cycle or because shes pregnant? i just dont know how this happened because when i wasnt home i made sure they were seperated and when she was runnin around the house she had her diaper on and i watched them outside. Can anyone help my baby cant have babies.