Gotten pregnant months after giving birth?

so, i just had a baby 5 months ago. now ive missed my period (they're -always- regular.) aswell as I'm having cramps, nausea, boob & back pain. I'm really freaking out. I tried to take a test today but no lines whatsoever showed up so I'm assuming it was defective. I'll be trying again tomorrow. I wanted to know if anyone else in here has gotten pregnant so soon after having a baby (i had a csection btw) and how it turned out for you? Im terrified tbh. We were using birth control but we did have sex on my fertile day (it was our wedding anniversary and I didnt even realize. we normally don't when I'm ovulating.) anyways, i just wanna know if other have gone through this and how it turned out for you. thank you!!