swollen left side neck gland, no fever, no sore throat

sorry but im desperate, im 14 and ive been experiencing a swollen gland on the left side of my neck for a week now, went to the doctor and he said that my throat was clear so was my ears, and no fever. ive been a little tired, but not exhausted, so why are they swollen? i had a sinus infection just recently, got medicine but didnt take the full ammount, i got better quite quickly.
ive been dealing with lower abdominal pain for a little while as well,  but i dont believe that its appart of the whole neck issue.
im just concerned because im scared i have HIV or something, i had unprotected sex about 4 months ago but we never really "finished it" if you know what i mean, he never came in me.
but could this be a case of low grade mono? or could it be signs of HIV?
please i would really appreciate your responses.
thank you.