First of all welcome to Yorkie Talk.
I have seen a lot of members on this forum that have experienced the same thing with their pregnant females. Is your female eating well? Sometimes the lower temperature is just due to low blood sugar. You can give her a little Nutri-cal or karo syrup and water and her temperature may come back up. Hopefully she will have them soon. Please keep us posted on how she is doing and best wishes for a safe whelping.