Fiance is Stressing Me Out!

We've been together for 2 years and now it seems like a non-stop fight. He recently returned from his 2nd tour to Afghanistan. Since he returned all we've done is fight. I spend most of the day afraid he'll blow up at me for denying to leave the house for cigaretts or fast food. 
On top of all that, I am over 50 days late with no symptoms of pregnancy. I took about 4 tests so far; all negative. I can't sleep, I gained 20 pounds, and I look tired daily. I don't know what to do. 
He's not taking his medication given to him for PTSD and anxiety. So he blows up at little things one minute and acts like nothing happened the next. 
I can't live like this, but I don't want to leave him since it's mostly not his fault.