From what you have described there appears to be no pregnancy
. The HCG levels are normal not raised.
I would like you to tell me if your periods are regular? If not then maybe there is a hormonal
reason why your uterus lining is thick.
as your ultrasound
is normal there seems to be no structural problem however certain conditions like PID or endometriosis
do not show up on ultrasound. That is because either the lesions are very small to show up on ultrasound or it is just inflammation of the reproductive organs which cannot be seen.
If your gynecologist suspects PID then he/she will prescribe some antibiotics and anti inflammatory agents for you. If your gynecologist suspects endometriosis then you may need a laproscopy to confirm the diagnosis. A few months of hormonal treatment may reduce your symtpoms.
hope this helps.