If archiving is enabled, this is the most likely explanation. At the start of each month, the Uptime Infrastructure Monitor data archiving process starts to remove old data from your DataStore (according to your data retention setting). This process can be very resource intensive and may take a long time to complete depending on the amount of historical data to be archived.
If this problem does not clear within an acceptable time frame, please contact IDERA Support to discuss an alternate data management plan and/or disable archiving and restart the up.time Data Collector service.