18 day period in July, and still waiting for another

Well, the title says it all. I was on the pill for 4 years. my periods are typically a few days, rarely over 4, then in July I had one that lasted 18 days. I saw a doctor and he said "next time double up on your pills." Well, then comes August, and I'm taking my pills as normal, and I never got my period. It happened again in September. I asked an older coworker, and she said maybe my hormones are off balanced, so I stopped taking my pills. i have yet to have one. thoughts? I'm not pregnant and have taken many tests.