Assuming normal anatomy (rare small percentage of people have their internal organs in reverse position from "normal"), the lower left abdomen contains the descending colon (large bowel), left ovary, and portions of the small bowel. Spleen is typically left upper quadrant while the liver & gall bladder are right upper quadrant. Down in the right lower abdomen is the ascending colon, appendix, right ovary, and portions of the small bowel. The stomach & pancreas can be thought of as centered in the upper half while the uterus & bladder are centered in the lower half (or pelvis). The kidneys are actually not in the abdomen but usually contribute to back pain if infected or if stones are present.
I mention "usually" & "typically" because it's not impossible (although improbable) for the appendix, liver, stomach, pancreas, etc to cause pressure & pain in the left lower quadrant. Bladder infections can do this but are usually associated w/change in your urinary habits, eg pain, frequency & urgency. Mittelschmerz is cyclic pain associated w/ovulation, so it tends to be one sided. Diverticulitis, which is an infection or at least inflammation of diverticulosis, is more likely to cause discomfort in the left lower quadrant. Constipation can do the same.
Endometriosis, which can be thought of as the lining of the uterus implanted outside the uterus, can attach itself to any part of the abdominal cavity and respond to hormones cyclically, just as if the endometrial tissue were still inside the uterus. When it comes time for your monthly menses, the blood can irritate that portion of your abdominal cavity. Of course, that same endometrial tissue could lead to adhesions, such that parts of your bowel (large & small) get stuck to each other. This is also a known complication of abdominal surgery.
Besides bladder infections, sexually transmitted infections of the Fallopian tube and/or ovarian structure, can cause one sided lower abdominal/pelvic pain. This is known as pelvic inflammatory disease or PID, and is diagnosed by cervical discharge and cervical motion tenderness as well as adnexal (tube & ovary) tenderness. A tubo-ovarian abscess is PID gone wild. Typically, you'd have many other symptoms and feel miserably, deathly ill, rather than just complain of pressure.
Uterine fibroids can increase pressure depending upon size & position as they grow. Depending upon your age & family history, we also worry about various cancers, too. The point is that you should have a chat w/your family physician about your concerns and get an appropriate evaluation. Good luck!