What would a lump be on the roof of my mouth. It's hard and only hurts when I press on it. It's been about a week now. What's the chances it's cancer?

Watch it. This could be something of inflammatory nature in which case it will settle down in 2 to 3 weeks. If it is still present or growing over the next 3-4 weeks, then you need to go see your doctor for a medical examination as it can be cancerous(if it is getting clearly bigger, yet it does not hurt).