3yr old temp went from 99.5 to 96.5 within a few hours. she had a low grade fever the past two days is it normal to drop so low. also has congestion?

It depends: Normal Body temperature can be between 97-98.6F. 96.5 is not that far off. Likely the drop was due to your child breaking his/her fever. Plus your thermometer might not be that accurate. A low temperature is considered below 95F. The congestion should improve over time. You could take multiple reads over the course of the day and average them if you want a more accurate reading.