It is totally normal and okay to feel like this, don't worry. Gender disappointment is a real thing one, and acknowledging your mixed feelings is the first step to process everything.
Give yourself some time to think, but don't feel guilty, your love for your son will naturally grow once you get ready to meet him.
Many parents also feel the same way at first, but later they find comfort seeing how loving a boy can be, or they just let go of expectations and take it day by day. Don't stress so much sis, you are not alone in this!