Would what I felt in my heart, and what one of my colleagues admitted to me regarding her backbiting about me, be considered ill-suspicion?

You are not permitted to assume the worst about your female colleagues without proof or evidence, for one of them admitting to something does not always mean it happened. Your gut feeling is not proof; rather, it is merely doubts, assumptions, and whisperings (waswas); therefore, seek refuge in Allah from them. We advise you to think well of your sisters and to have a pure heart towards them, for this is one of the best acts of obedience.