Me and my partner had sex for the first time a month ago, and were positive that we were protected and nothing went though the condom but since then I've felt my stomach become more firm but I can't still suck it in and its still a little fatty (im a small girl) . I have and irregular period so I don't know when I should get it but I'm getting a little stressed about all of this
Could I be pregnant? I'm getting no other symptoms (nausea, backpain, breast pain) so if there any possible way I could be pregnant or am I just paranoid?
Also I took a pregnancy test a week ago and it said negative, my last period was November 12, and its December 12 now, but it usually takes 40 days for my cycle.

If you have sex then there is a chance of pregnancy regardless of the protection you use. There are people were that can tell you about having used three types of birthcontrol properly and how they got pregnant. That being said birth control reduces the chances of that happening significantly. Condoms is the least protective at about 80-90% provided that they're used correctly.

Are you pregnant? I can't say. Until month two or three your uterus is still sitting nicely in your pelvis. So your belly being firm is mostly psychosomatic at this point. You're so concerned that you're taking every sign as being pregnant instead of ignoring them like you usually do.

What you need to do is wait until you miss your period by two weeks. You say it is a 40 day cycle, so that would put you in the first weekend of January. Then take a home pregnancy test with early morning urine. There is a high probability of getting a false negative if you test sooner.

Good Luck!