It's unlikely that you have ovarian cancer at 17. Cysts on the ovaries are also VERY common and often are not a concern.
Do see your gynecologist and do tell them EVERYTHING. You are not a hypochondriac, just concerned about your health. If you find it easier to put things in writing before you see your doctor, do so.
Just an FYI, frequently taking OTC medications CAN CAUSE you to have bowel problems. Your body becomes used to them.
Periods can often be irregular in your teens. Expect some blood work to check your hormone levels to rule that out - it is a common cause. Often birth control pills are used to help regulate your cycle, depending upon what they find.
Hope it helps.