Your risk is at or near zero.
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Safe sex sex and STDs are not transmitted that way. Similarly, for a person to touch your anus is of no risk to you. Finally, the time interval you describe is so long ago that any STD that might have been there likely would have been cleared by your own host defenses. STDs are not something you need to worry about, based on your description.
Regarding your questions:
While the symptoms you describe could be caused by rectal STDs, your exposure history is in no way compatible with this. Proctitis and rectal irritation has many causes, ranging from something as simple as hemorrhoids to chronic illnesses such as inflammatory bowel disease. The best way to sort this out is to have your problems evaluated by an experienced clinician.
Keflex has no effect on trichomoniasis.
Hope this helps. STD risk-zero. See a clinician for evaluation of other causes. EWH