So I have checked the page with W3C validator and got the following error:
Line 107, Column 56: Element ul not allowed as child of element span in this context. (Suppressing further errors from this subtree.)
<span class=”category”><ul class=”post-categories”>
I suspect this is the cause as the error shows the “Categories” title going up to the nav bar while it should have been on the sidebar. I’m not sure where this span class came from as I applied <div> tags in the dynamic sidebar function.