hi welcome to HCM
-i had gone through your query and understand your concerns. i would come up with two possibilities.
1.the first one may be ENDOMETRIOSIS;thickness of the endometrium ;ultrasound
scanning for pelvis may confirm the diagnosis.
2.the second one is, ACID PEPSIC DYSPEPSIA
possible causes include;
-general poor health
-eating junk food
-taking pain killers etc
I hope this is helpful for you, thank you