Please don't worry!! You used language in a way that was entirely understandable and simply mistaken. That is not a sin. It only becomes a sin if you are corrected by someone truly knowledgeable and you persist in misusing the language for unworthy motives. Then you might become culpable for actual sin, not because you misuse the title per se (though you would then be guilty of being fraudulent or a counterfeit "Catholic Hermit"), so much as because of accompanying stubbornness, arrogance, a lack of charity (it is uncharitable to knowingly misrepresent oneself in this way), and disobedience or other such motives. The fact that we are baptized gives us the right to call ourselves Catholic, but we cannot call ourselves a Catholic Hermit because, as I have already explained here, that means a Hermit specifically representing this vocation in the name of the Church. It would be similar to a situation in which you are a teacher and a Catholic, but not a Catholic teacher (an official catechist) who teaches in the name of the Church, not in her own name.**
No, you are not an illegal hermit. I cannot emphasize that enough. You are a Catholic and a hermit living that calling by virtue of the grace and freedom of your baptism. You have every right to do that. In October 1983 the Church added c 603 to allow for solitary consecrated hermits for the first time in universal law. It became, on that day, the norm (canon) for solitary Catholic Hermits. Because you do not fall under that canon (norm) or under the canons applying to religious men and women living semi-eremitical lives, you are a "non-canonical" hermit. Those of us who do fall under these canons in addition to those that apply to every Catholic by virtue of baptism, are called "canonical". This also means that you are not called to live your hermit life as normative of the Church's understanding of this vocation. Those who live canonical hermit lives are called to publicly represent a normative form of eremitical life. This does not mean, by the way, that they are perfect hermits nor that they are identical to one another, but rather, that they understand this vocation as the Church does, have petitioned and been admitted to a public commitment that will deepen and intensify in time, that their call is an ecclesial vocation that proclaims the Gospel of Jesus Christ and the hope it brings to both Church and World. (It does this latter because the inner journey she has made (and is committed to making ever more deeply) allows her to experience both death and resurrection in her encounter with Mystery.)If you, and others of course, should discover that your own eremitical life is moving in this direction, you might then consider whether you feel called to a canonical commitment. Even if you do not move in this direction, you can continue to live freely as a non-canonical hermit based on the grace and freedom of your baptism. Please don't let anyone convince you that that makes you an "illegal" hermit! That inaccurate and unnuanced form of language is denigrating to the lay hermit vocation!!
By the way, you should be aware that if you do something wrong, or see someone else doing something wrong, even something the Church identifies as an intrinsic evil (like killing someone), only the one doing the wrong and God can say whether a sin has been committed. I have heard of someone opining on the hermit life and speaking wrongly about others' sins, or even about (her own) "being an occasion of sin for others". In fact, to speak this way, to suggest that another person is sinning or has sinned because one knows what actions they took, is an impermissible judgment. We can know that the person's behavior (the action they took) is wrong, unworthy of being chosen, and even intrinsically evil, for instance, but we cannot know whether they have sinned unless they tell us they have. (I am assuming God is NOT going to tell us such a thing!!)


