Dear LaCat: There may not be the same cause for each of your symptoms so let's take each symptom one by one, OK?
Midcycle spotting:If your spotting begans about cycle day 14 (cycle day 1 being the first day of your last period), this specific timing suggests the most likely cause would be bleeding with ovulation. Right before ovulation there is a brief spike of estrogen. When this level drops back down to normal, the sharp decline can destabilize the lining of the uterus leading to spotting/bleeding. Some women have this sign of ovulation every month; others only rarely. Fortunately the amount is usually scant and brief in duration.
Some other possible causes of erratic spotting can include a new chlamydia infection (unlikely if you are both monogamous). polyps of the lining of the uterus (way more common in midlife women), or missed/erratic ovulations (usually there will be a history of missed/irregular periods).
Acne: This can be prompted by an increase in male hormones (androgens). The most common cause for a young women to have increased androgen levels in "Polycystic Ovarian Syndrome" known as PCOS. Both ovaries have multiple small cysts and a change in part of the ovarian tissue which allows androgen levels to rise. Also associated with PCOS is being heavy set, and having unwanted hair growth.
Severe menstrual cramping: Worsened cramping can be linked to heaver flows (increases prostaglandin levels). Increased cramping over times can be a sign of endometriosis. This later condition would be more likely if there is a family history of endometriosis.
Left lower abdominal pain with exercise---While a left ovarian cyst can certainly trigger this type of pain there are some other causes as well. If endometriosis is actually present it can create "adhesions" (filmy bands of scar tissue which "web together" pelvic organs). Even the intestine can be a source of cramping pain.
Bottom line, the spotting and acne may be hormonal (ie related to the ovarian hormones) as can an increasingly heavy flow. Your MD or GYN can give you the most accurate diagnosis based upon your personal and family health histories.
Yours,
Jane