Typically levels will double every two days, so this is a bit lower than expected. Is your doctor continuing to monitor and test your levels? There are many times I have heard of that levels don't double as quickly as expected- but everything ends up being fine. It can be a bit of a warning sign, but at this point it's something just to watch. The hard part is the waiting it- so hang in there. Your levels are going up, so up is a good direction. I'd request another test of your HCG levels, just to get a bit more information and to continue to monitor things.
Let us know how you are doing!