If I was christened am I not Catholic? I was christened as an infant in the Catholic faith. Due to where I lived, a Catholic church was not available, so I attended a Baptist church. I married a Catholic man, when we went to mass with his family, I was told by his family not to partake. It offended me but were they in the right?
I would say they erred on the side of caution. Your baptism is fully valid in the Catholic Church and since you are mature enough to distinguish the Eucharist from regular food you have every right to partake.

However, because you were raised in a Baptist church, you did not receive a Catholic education nor did you have a Catholic confirmation. (In fact the Baptists do not Confirm since they do not baptize infants.) I also assume that you did not perform the sacrament of penance before taking Communion. Therefore, you partaking could present the appearance of impropriety. If you intend to continue worshiping in the Catholic church you should discuss taking the Rite of Christian Initiation for Adults with a priest. This will include education, Penance, Confirmation, and Communion.