If she is getting a shot, it's less likely that it is messing with her moods. As far as sexual enhancers, she would not likely want to use them because they could interfere with the BC.
She is most likely suffering from depression, including post-partum depression, which can last long after a baby is born. She is all lone, every day, while you are deployed and has to raise 2 kids most of the time on her own. That's a challenge. She probably needs rest and attention.
If she refuses to get help, you can't make her get it, but it is advisable. However, you are not being supportive of what she is going through and understanding her either. You are mostly putting down the fact that you have been away and have needs and expect them fulfilled when you return home after a hard time with the military. I know that it is very difficult to see the things that you do over in no-mans-land, but she is going through it with you with the knowledge that one time you may not come back either, and then she will be completely alone.
Knowing that a divorce would be something of her own power, it would be easier to cut you off than to have it in the hands of fate. It gives her control.
If you both want to work things out, but argue every time you talk, then don't talk. That makes matters worse, plus it isn't healthy for the kids. I find that a very effective method is writing letters or emailing back and forth until the issue is resolved. This helps people say what they need without the other flaming up immediately at what is said, and it also makes the other listen to what the other has to say before shouting out a rebuttal.
If you cannot solve it in this way, then maybe you can see a marriage counselor.