Is there over-the-counter medicine that I can take for diverticulitis?

No. Diverticulosis is a process through which the colon makes "pockets" over time. It is a common condition and generally does not cause discomfort or fever. However, when one of these pockets becomes obstructed through stool or food by-products, the condition is called diverticulitis. This is a more serious condition for which antibiotics, either orally or intravenously administered, are required.
No. You should see a physician for diverticulitis to prevent it from progressing. Over the counter should not be sought without a doc's input.