Your paper is yellowing because it has wood-pulp content.
To avoid this happening in the future you should try the more expensive 100% rag content paper. Paper made from cotton fibers will be stronger and not yellow so quickly.
Also, wood-pulp papers tend to have a lot of bleach in them. While this gives the paper a nice white finish, the bleach will attach your pigments over time. Colors will be fugitive and even india ink can be attacked.
If you are doing sketches or work not intended to last, use the wood-pulp papers for that. Anything you intend to last for a decade or more should be done on 100% rag content paper. Strathmore 500 series paper is 100% rag.