Short answer. To the conjugal partnership. Article 119 of the Family Code provides that while installment payments collected during the marriage on a pre-marriage credit remain the exclusive property of the owning spouse, interest falling due on that principal during the marriage belongs to the conjugal partnership instead.
What the law says
Whenever an amount or credit payable within a period of time belongs to one of the spouses, the sums which may be collected during the marriage in partial payments or by installments on the principal shall be the exclusive property of the spouse. However, interests falling due during the marriage on the principal shall belong to the conjugal partnership.
Family Code, Article 119 — Credit Payable in Installments. Read the full provision →
Interest goes to the conjugal partnership
Article 119 states plainly that interests falling due during the marriage on the principal shall belong to the conjugal partnership. Since the debt owed to you predates your marriage, any interest that accrues and falls due on that debt while you are married goes to the conjugal partnership, not to you individually, regardless of the fact that the underlying debt itself is yours alone.
The principal installments stay yours
The same article treats the principal differently: the sums which may be collected during the marriage in partial payments or by installments on the principal shall be the exclusive property of the spouse. So the two streams of money coming from the same debt are split — the installment payments reducing the principal remain yours exclusively, while the interest earned on that principal during the marriage goes to the conjugal partnership instead.
Why the article separates principal from interest
The article's structure reflects a distinction between the capital itself and what it generates. The credit belonged to you before the marriage, so the underlying principal continues to trace back to your separate ownership as it is collected. Interest, though, is treated as a fruit the marriage's existence helped generate during the time it accrued, which is why the article routes it to the conjugal partnership rather than to you individually.
What the article does not address
Article 119 fixes where the principal and the interest go once collected, but it does not itself address how the interest rate or amount is calculated, or what happens if the debtor pays principal and interest together without clearly separating the two. Those practical questions depend on the terms of the underlying debt and the records kept of what was actually collected and when. Nor does it decide the fate of interest that fell due before the wedding day: the article's rule is written for interests falling due during the marriage, so the timing of each amount is the fact worth documenting.