late period

Okay so my boyfriend is in the military, and the night before he left to go to BMT, the condom came off like halfway through, so he wiped off, and we didn't use one the rest of the time, and I made sure he pulled out. My period is now a week late, and two pregnancy tests came up negative. I'm really worried because this isn't the time in his career for this, and I'm also only 19. I can't write and tell him I'm worried because I don't want to distract him from his training. Advice??