You may have pick up an automatic relation somewhere like perpendicular or parallel.
3D sketches can be touchy. Just how it is.
I turn off all automatic relations when making 3D sketches. Just plain easier than hunting down accidental auto relations. Also forces you to be more deliberate with your actions.
3D sketching also challenges your assumptions about constraints and geometry. Think twice about each of your constraints and what they actually do (and don't do). Sometimes there's a solution that seems like the long way around that is actually more robust.
Often I find it is too much to put everything into a single sketch. Many times I create a "foundation" sketch to anchor key elements of the final 3D sketch. Especially true when working with splines.
Thanks guys, I'll give that a try. I hadn't considered disabling the auto-relations.