If you went to the hospital with stomach pains and a UTI/Bladder/Yeast infection they should have taken a blood sample. That blood sample tests for everything and they should have detected a pregnancy if there was one. I would say most likely its just a weird cycle.
Just because you felt ovulation pain,this dont mean you did infact ovulate.our bodys struggle to release the egg somtimes.so we assume we are late when in fact ovulation happened later,if at all.making a later period.I personally would say at nearly 2 weeks late ,and negative test,then your not pregnant and that you havent ovulated yet.as previous poster said your luteal phase is never more than 16 days. no matter what.so i would agree have a blood test,but as i say im guessing you havent ovulated yet.if you feel your too young yet for a baby try some form of birth control.im not judgeing you at all,i myself was 19 when I had my son,it can be done,but if your not ready,use BC hun,best of luck.
If you take another test make sure you haven't pee'd for 4 hours (I read somewhere that if your early in it helps make sure the HCG isn't diluted). Also, you may want to set up an apt with your dr. It's possible you just don't have enough hormone in you yet. A blood test can determine how much is in you, I believe most HPC's need at lest 25, but 10 or something is when they consider your pregnant.
Stress can cause you to be late, but it causes you to ovulate late so if your stressing AFTER you ovulate you should not be late. The amount of time from Ovulation to AF should be exactly the same for every cycle for you. (For me it's 13 days, average is 14 days).
Even if you are pregnant, many people still go to school while pregnant. I went to an all girls college and there were a lot of adults, and a lot of women who were either pregnant or just had babies. Having a baby won't ruin everything, it may set you back a little bit.
I hope the best for you!
No one can really tell you what is going on but if the problem persists then you need to go see your doctor again and express your concerns.