First this must be a frightening situation for you and your fiance to be in.
Your verb choice of "say I WAS". Reality check that - This is not a hypothetical - you either ARE or your are not pregnant. I recommend it's time now for you and your fiance as a team to take a couple of actions.
First, agree strongly with recommendation for counseling/ support - your school should have counseling, there is planned parenthood, or let the group know if you two need any support in finding someone to talk to.
Recommend you take a pregnancy test so you are both informed. If it has been "a couple of weeks" since you both completed the act of making a child, as you stated- your body only needs 7-12 days (already past) to develop detectable levels of HCG after the egg is implanted, so you may be able to get an early indicator today. For the most accurate results, you should take a pregnancy test the week after your missed period. I think the tests come in 2-packs - If it were me, I would do two tests today, and another two a week after your missed period, if that happens. the cost of those kits is less that even a week of diapers/formula. Make the investment.
After this... I didn't want chicken last night for dinner, so I just chose not to make it - your choices have bigger consequences. It's unfortunate if you do not want a child that you didn't take a plan B bill within a short window after unprotected sex - if you had we would likely never know if you would have been pregnant. Learn about that for "next time". For the future, too, another option - if you and your fiance are sexually active and you don't want a child, why not take birth control - aka "the pill"? Well after this crisis think about all that.
Given that surgical abortions are a DQ per the other post from a national expert, just please know there are non invasive / non procedural medication options to ending a pregnancy if that is your choice. You both can discuss options and risks with a physician or planned parenthood. There is also adoption but I am trying to stay out of sharing my opinion or a recommendation - just noting it's an option.
For AROTC per the text below, it sounds like you would not be disenrolled for the "event" of pregnancy, but I am not sure what branch you are in. Make sure you understand - here from the internet was "Pregnant students are not ineligible to compete for scholarship. However, they are ineligible to enroll in ROTC if pregnant. Therefore, students must be medically qualified at the time of enrollment. Cadets who become pregnant after enrollment will not be involuntarily disenrolled solely because of pregnancy."
good luck and let the group know if any support would be helpful. If you must end a pregnancy please do not let this fester/ go on for months. Be as humane and responsible as you can now.
On informing the cadre about a pregnancy you aren't keeping? I don't know. Maybe this is an area where a woman has a right to privacy vs having this as part of your file. The only times I've lied in the last 20 years are when I'm asked "do I look fat in this dress?" by some women in my life who sometimes indeed do look fat in those dresses.I enjoy the freedom and simplicity / ease of just being truthful. Cherish it actually. But in this case, I just don't know. Is there a way to disclose the pregnancy and just say not brought to term vs. aborted? does that matter? I'd avoid having abortion as part of my DDs record if possible -it's a "NUNYA" business detail IMO.
Hope this helps and know this will work out - if you are overwhelmed/ suicidal thoughts come in here, at all seek immediate help please. You will get through this., learn from it hopefully, and good luck.