In that case, you have probably covered basic vector calculus (the Gibbs formulation). You should probably study real analysis, which is the rigorous theory behind calculus, and the calculus of complex variables. These would both prepare you for rigorous courses in differential equations.
A nice segue into real analysis is Spivak's text "Calculus", as it is rigorous where most other texts are not, and a very friendly read compared to some dense analysis texts, as it bases most of its theorems on ideas you already know from your calculus courses, but with more detail. A proper analysis text (that is, one that covers the fundamentals like measure theory and topology) would be Rudin's "Principles of Mathematical Analysis". If you have taken a proper theory-based course of linear algebra using proofs, you should be ready to work through this.