my sis in law ended up pregnant because of the anti biotics she was on, they interfered with the pill, the same happened with a woman's daughter in law i work with though she miscarried on her 8th week. So meds can interfere with your system and most doctors will tell you that before they give you any medication. Again i hope you get the result you are looking for
Any time that you have unprotected sex, you can get pregnant. Retest again in a week. If you still think you might be pregnant at that time, go to the doctors for a blood test.
Illness and stress can effect your cycle. Most medications will not.
the fact you never had intercourse 14 days after the first day of your cycle makes me believe you are not pregnant as you state you had intercourse on the 05th and 08th Oct meaning only 8days after day 1 of your cycle.
I think the meds the docs gave you for the infection have knocked your cycle a little off course, i would test again and if you miss another period i would speak to a doc, best of luck to you