@confused123,
Hamlet would say yes.... but things didn't really work out that great for him.
It really depends on what you mean by "right" to feel that way.
It's understandable I think. Heck it's the basis of a major Shakespearean tragedy (well that and that the uncle killed the father, but initially Hamlet is just disgusted that his mother and his uncle are sleeping together so soon after his father's death).
The question is whether your understandable "ewww" reaction means that you're justified in refusing to forgive your mother.
That part's not right, I don't think. As unpleasant as it may be for you to think about -- and we usually find thinking about our parents' sex lives unpleasant -- she is not doing anything wrong. She's already lost her husband (how long ago?), having her daughter mad at her can't be easy.
Not to mention that it must be hard for you to have lost your father and now be having such a hard time with your mom.