I'm a bit confused

Hello. I am having quite a few problems. I first want to say that I had sex in the beginning and toward the end of October. I had no period that month. November I had light spotting but intense pain. Same for December and this month. I took a couple hpts and the first couple were negative. The rest was positive. I went to the doctor for a blood test and it was negative. I then asked for an ultrasound. Couldn't actually get one unless my blood test showed up positive. So I told them I thought I had an ovarian cyst. Got a call back about an ultrasound right away. Truth is, my Ob actually did think I was pregnant but she had to follow procedure. At the ultrasound, they actually did find a cyst but no baby. My faith of having a baby was gone. I figured because I would've been 14 weeks pregnant and I'm pretty sure you be able to see it very clearly. My question is, can sex throw you off that much that you never have a regular period ever again? All I get is spitting. And all of the pregnancy signs are present. I'm not too disappointed but I do want to know what's actually going on. Is there a chance I could be pregnant and the ultrasound just couldn't read it? Has anyone else gone through this? I would love all feedback, remember I'm not trying to be pregnant. I'm still young. But I want to know if I am okay.